Method of producing non-flammable quasi-solid electrolyte and a quasi-solid electrolyte/separator layer for use in a lithium battery

ABSTRACT

Provided is a method of producing a non-flammable quasi-solid electrolyte for a lithium battery, the method comprising (A) dissolving a lithium salt in a first liquid solvent to obtain a mixture having a first concentration of lithium salt less than 3.0 M (mole/L), but greater than 0.001M; and (B) removing a portion of the first liquid solvent to obtain the quasi-solid electrolyte having a final lithium salt concentration higher than the first concentration so that the electrolyte exhibits a vapor pressure less than 0.01 kPa when measured at 20° C., a vapor pressure less than 60% of the vapor pressure of the first liquid solvent alone, a flash point at least 20 degrees Celsius higher than a flash point of the first liquid solvent alone, a flash point higher than 150° C., or no detectable flash point.

The present disclosure provides a non-flammable electrolyte compositionand electrolyte/separator layer for use in a secondary or rechargeablelithium battery and a method for producing the electrolyte composition,electrolyte/separator assembly, and the lithium battery.

BACKGROUND

Rechargeable lithium-ion (Li-ion), lithium metal, lithium-sulfur, and Limetal-air batteries are considered promising power sources for electricvehicle (EV), hybrid electric vehicle (HEV), and portable electronicdevices, such as laptop computers and mobile phones. Lithium as a metalelement has the highest lithium storage capacity (3,861 mAh/g) comparedto any other metal or metal-intercalated compound as an anode activematerial (except Li_(4.4)Si, which has a specific capacity of 4,200mAh/g). Hence, in general, Li metal batteries (having a lithium metalanode) have a significantly higher energy density than lithium-ionbatteries (having a graphite anode).

Historically, rechargeable lithium metal batteries were produced usingnon-lithiated compounds having relatively high specific capacities, suchas TiS₂, MoS₂, MnO₂, CoO₂, and V₂O₅, as the cathode active materials,which were coupled with a lithium metal anode. When the battery wasdischarged, lithium ions were transferred from the lithium metal anodeto the cathode through the electrolyte and the cathode became lithiated.Unfortunately, upon repeated charges and discharges, the lithium metalresulted in the formation of dendrites at the anode that ultimatelycaused internal shorting, thermal runaway, and explosion. As a result ofa series of accidents associated with this problem, the production ofthese types of secondary batteries was stopped in the early 1990'sgiving ways to lithium-ion batteries.

Even now, cycling stability and safety concerns remain the primaryfactors preventing the further commercialization of Li metal batteries(e.g. Lithium-sulfur and Lithium-transition metal oxide cells) for EV,HEV, and microelectronic device applications. Again, cycling stabilityand safety issues of lithium metal rechargeable batteries are primarilyrelated to the high tendency for Li metal to form dendrite structuresduring repeated charge-discharge cycles or overcharges, leading tointernal electrical shorting and thermal runaway. This thermal runawayor even explosion is caused by the organic liquid solvents used in theelectrolyte (e.g. carbonate and ether families of solvents), which areunfortunately highly volatile and flammable.

Many attempts have been made to address the dendrite and thermal runawayissues. However, despite these earlier efforts, no rechargeable Li metalbatteries have succeeded in the marketplace. This is likely due to thenotion that these prior art approaches still have major deficiencies.For instance, in several cases, the anode or electrolyte structuresdesigned for prevention of dendrites are too complex. In others, thematerials are too costly or the processes for making these materials aretoo laborious or difficult. In most of the lithium metal cells andlithium-ion cells, the electrolyte solvents are flammable. An urgentneed exists for a simpler, more cost-effective, and easier to implementapproach to preventing Li metal dendrite-induced internal short circuitand thermal runaway problems in Li metal batteries and otherrechargeable batteries.

Parallel to these efforts and prompted by the aforementioned concernsover the safety of earlier lithium metal secondary batteries led to thedevelopment of lithium-ion secondary batteries, in which pure lithiummetal sheet or film was replaced by carbonaceous materials (e.g. naturalgraphite particles) as the anode active material. The carbonaceousmaterial absorbs lithium (through intercalation of lithium ions or atomsbetween graphene planes, for instance) and desorbs lithium ions duringthe re-charge and discharge phases, respectively, of the lithium-ionbattery operation. The carbonaceous material may comprise primarilygraphite that can be intercalated with lithium and the resultinggraphite intercalation compound may be expressed as Li_(x)C₆, where x istypically less than 1.

Although lithium-ion (Li-ion) batteries are promising energy storagedevices for electric drive vehicles, state-of-the-art Li-ion batterieshave yet to meet the cost, safety, and performance targets. Li-ion cellstypically use a lithium transition-metal oxide or phosphate as apositive electrode (cathode) that de/re-intercalates Li⁺ at a highpotential with respect to the carbon negative electrode (anode). Thespecific capacity of lithium transition-metal oxide or phosphate basedcathode active material is typically in the range from 140-170 mAh/g. Asa result, the specific energy of commercially available Li-ion cells istypically in the range from 120-220 Wh/kg, most typically 150-180 Wh/kg.These specific energy values are two to three times lower than whatwould be required for battery-powered electric vehicles to be widelyaccepted.

Furthermore, the same flammable solvents previously used for lithiummetal secondary batteries are also used in most of the lithium-ionbatteries. Despite the notion that there is significantly reducedpropensity of forming dendrites in a lithium-ion cell (relative to alithium metal cell), the lithium-ion cell has its own intrinsic safetyissue. For instance, the transition metal elements in the lithium metaloxide cathode are highly active catalysts that can promote andaccelerate the decomposition of organic solvents, causing thermalrunaway or explosion initiation to occur at a relatively low electrolytetemperature (e.g. <200° C., as opposed to normally 400° C. without thecatalytic effect).

Ionic liquids (ILs) are a new class of purely ionic, salt-like materialsthat are liquid at unusually low temperatures. The official definitionof ILs uses the boiling point of water as a point of reference: “Ionicliquids are ionic compounds which are liquid below 100° C.”. Aparticularly useful and scientifically interesting class of ILs is theroom temperature ionic liquid (RTIL), which refers to the salts that areliquid at room temperature or below. RTILs are also referred to asorganic liquid salts or organic molten salts. An accepted definition ofan RTIL is any salt that has a melting temperature lower than ambienttemperature.

Although ILs were suggested as a potential electrolyte for rechargeablelithium batteries due to their non-flammability, conventional ionicliquid compositions have not exhibited satisfactory performance whenused as an electrolyte likely due to several inherent drawbacks: (a) ILshave relatively high viscosity at room or lower temperatures; thus beingconsidered as not amenable to lithium ion transport; (b) For Li—S celluses, ILs are capable of dissolving lithium polysulfides at the cathodeand allowing the dissolved species to migrate to the anode (i.e., theshuttle effect remains severe); and (c) For lithium metal secondarycells, most of the ILs strongly react with lithium metal at the anode,continuing to consume Li and deplete the electrolyte itself duringrepeated charges and discharges. These factors lead to relatively poorspecific capacity (particularly under high current or highcharge/discharge rate conditions, hence lower power density), lowspecific energy density, rapid capacity decay and poor cycle life.Furthermore, ILs remain extremely expensive. Consequently, as of today,no commercially available lithium battery makes use of an ionic liquidas the primary electrolyte component.

With the rapid development of hybrid (HEV), plug-in hybrid electricvehicles (HEV), and all-battery electric vehicles (EV), there is anurgent need for anode and cathode materials and electrolytes thatprovide a rechargeable battery with a significantly higher specificenergy, higher energy density, higher rate capability, long cycle life,and safety. One of the most promising energy storage devices is thelithium-sulfur (Li—S) cell since the theoretical capacity of Li is 3,861mAh/g and that of S is 1,675 mAh/g. In its simplest form, a Li—S cellconsists of elemental sulfur as the positive electrode and lithium asthe negative electrode. The lithium-sulfur cell operates with a redoxcouple, described by the reaction S₈+16Li↔8Li₂S that lies near 2.2 Vwith respect to Li⁺/Li^(o). This electrochemical potential isapproximately ⅔ of that exhibited by conventional positive electrodes.However, this shortcoming is offset by the very high theoreticalcapacities of both Li and S. Thus, compared with conventionalintercalation-based Li-ion batteries, Li—S cells have the opportunity toprovide a significantly higher energy density (a product of capacity andvoltage). Values can approach 2,500 Wh/kg or 2,800 Wh/l based on thecombined Li and S weight or volume (not based on the total cell weightor volume), respectively, assuming complete reaction to Li₂S. With aproper cell design, a cell-level specific energy of 1,200 Wh/kg (of cellweight) and cell-level energy density of 1,400 Wh/l (of cell volume)should be achievable. However, the current Li-sulfur products ofindustry leaders in sulfur cathode technology have a maximum cellspecific energy of 400 Wh/kg (based on the total cell weight), far lessthan what could be obtained in real practice.

In summary, despite its considerable advantages, the rechargeablelithium metal cell in general and the Li—S cell and the Li-air cell inparticular are plagued with several major technical problems that havehindered its widespread commercialization:

-   (1) Conventional lithium metal secondary cells (e.g., rechargeable    Li metal cells, Li—S cells, and Li-Air cells) still have dendrite    formation and related internal shorting and thermal runaway issues.    Also, conventional Li-ion cells still make use of significant    amounts of flammable liquids (e.g. propylene carbonate, ethylene    carbonate, 1,3-dioxolane, etc) as the primary electrolyte solvent,    risking danger of explosion;-   (2) The Li—S cell tends to exhibit significant capacity degradation    during discharge-charge cycling. This is mainly due to the high    solubility of the lithium polysulfide anions formed as reaction    intermediates during both discharge and charge processes in the    polar organic solvents used in electrolytes. During cycling, the    lithium polysulfide anions can migrate through the separator and    electrolyte to the Li negative electrode whereupon they are reduced    to solid precipitates (Li₂S₂ and/or Li₂S), causing active mass loss.    In addition, the solid product that precipitates on the surface of    the positive electrode during discharge can become electrochemically    irreversible, which also contributes to active mass loss.-   (3) More generally speaking, a significant drawback with cells    containing cathodes comprising elemental sulfur, organosulfur and    carbon-sulfur materials relates to the dissolution and excessive    out-diffusion of soluble sulfides, polysulfides, organo-sulfides,    carbon-sulfides and/or carbon-polysulfides (hereinafter referred to    as anionic reduction products) from the cathode into the rest of the    cell. This phenomenon is commonly referred to as the Shuttle Effect.    This process leads to several problems: high self-discharge rates,    loss of cathode capacity, corrosion of current collectors and    electrical leads leading to loss of electrical contact to active    cell components, fouling of the anode surface giving rise to    malfunction of the anode, and clogging of the pores in the cell    membrane separator which leads to loss of ion transport and large    increases in internal resistance in the cell.

SUMMARY

In response to these challenges, new electrolytes, protective films forthe lithium anode, and solid electrolytes have been developed. Someinteresting cathode developments have been reported recently to containlithium polysulfides; but, their performance still fall short of what isrequired for practical applications. Despite the various approachesproposed for the fabrication of high energy density rechargeable cellscontaining elemental sulfur, organo-sulfur and carbon-sulfur cathodematerials, or derivatives and combinations thereof, there remains a needfor materials and cell designs that (a) retard the out-diffusion ofanionic reduction products, from the cathode compartments into othercomponents in these cells, (b) improve the battery safety, and (c)provide rechargeable cells with high capacities over a large number ofcycles.

Again, lithium metal (including pure lithium, alloys of lithium withother metal elements, or lithium-containing compounds) still providesthe highest anode specific capacity as compared to essentially all otheranode active materials (except pure silicon, but silicon haspulverization issues). Lithium metal would be an ideal anode material ina lithium-sulfur secondary battery if dendrite related issues, such asfire and explosion danger, could be addressed. In addition, there areseveral non-lithium anode active materials that exhibit high specificlithium-storing capacities (e.g., Si, Sn, SnO₂, and Ge as an anodeactive material) in a lithium ion battery wherein lithium is insertedinto the lattice sites of Si, Sn, SnO₂, or Ge in a charged state. Thesepotentially useful anode materials have been largely ignored in theprior art Li—S cells.

Hence, a general object of the present disclosure is to provide anelectrolyte-separator system for a rechargeable lithium cell thatexhibits a high energy density, high power density, long cycle life, andno danger of explosion due to the use of a safer, non-flammable,quasi-solid electrolyte. The disclosure also provides a process forproducing this non-flammable electrolyte-separator layer in a separatesheet form or in a roll form. The electrolyte-separator layer can beused in a broad array of rechargeable lithium cells, including, forinstance, the lithium metal secondary cell (e.g. Li—S, Li—TiS₂, Li—MoS₂,Li—VO₂, and Li-air, just to name a few), lithium-ion cell (e.g.graphite-LiMn₂O₄, Si—Li_(x)Ni_(y)Mn_(z)O₂, etc.), Li-ion sulfur cell(e.g. prelithiated Si—S cell), and hybrid lithium cell (wherein at leastone electrode operates on lithium insertion or intercalation).

A specific object of the present disclosure is to provide anon-flammable, solid-like electrolyte that enables a rechargeable Li—Sbattery to exhibit an exceptionally high specific energy or high energydensity and a high level of safety. One specific technical goal of thepresent disclosure is to enable the construction of a safe Limetal-sulfur or Li ion-sulfur cell having a long cycle life and a cellspecific energy greater than 500 Wh/kg, preferably greater than 600Wh/kg, and more preferably greater than 800 Wh/kg (all based on thetotal cell weight).

Another specific object of the present disclosure is to provide anelectrolyte-separator system that enables a safe lithium-sulfur cellexhibiting a high specific capacity (higher than 1,200 mAh/g based onthe sulfur weight, or higher than 1,000 mAh/g based on the cathodecomposite weight, including sulfur, conducting additive and conductivesubstrate, and binder weights combined, but excluding the weight ofcathode current collector). The specific capacity is preferably higherthan 1,400 mAh/g based on the sulfur weight alone or higher than 1,200mAh/g based on the cathode composite weight. This must be accompanied bya high specific energy, good resistance to dendrite formation, goodresistance to thermal runaway, no possibility of an explosion, and along and stable cycle life.

It may be noted that in most of the open literature reports (scientificpapers) on Li—S cells, scientists choose to express the cathode specificcapacity based on the sulfur weight or lithium polysulfide weight alone(not on the total cathode composite weight), but unfortunately a largeproportion of non-active materials (those not capable of storinglithium, such as conductive additive and binder) is typically used intheir Li—S cells. Similarly, for lithium-vanadium oxide cells,scientists also tend to report the cathode specific capacity based onthe vanadium oxide weight only. For practical usage purposes, it is moremeaningful to use the cathode composite weight-based capacity value.

A specific object of the present disclosure is to provide anelectrolyte-separator layer, the production process, and the associatedrechargeable lithium-sulfur cell based on rational materials and batterydesigns that overcome or significantly reduce the following issuescommonly associated with conventional Li—S cells: (a) dendrite formation(internal shorting); (b) extremely low electric and ionic conductivitiesof sulfur, requiring large proportion (typically 30-55%) of non-activeconductive fillers and having significant proportion of non-accessibleor non-reachable sulfur or lithium polysulfides); (c) dissolution oflithium polysulfide in electrolyte and migration of dissolved lithiumpolysulfides from the cathode to the anode (which irreversibly reactwith lithium at the anode), resulting in active material loss andcapacity decay (the shuttle effect); and (d) short cycle life.

Another object of the present disclosure is to provide a simple,cost-effective, and easy-to-implement approach to preventing potentialLi metal dendrite-induced internal short circuit and thermal runawayproblems in various Li metal and Li-ion batteries.

The present disclosure provides a method of producing a non-flammablequasi-solid electrolyte for a lithium battery, the method comprising (A)dissolving a lithium salt in a first liquid solvent to obtain a mixturehaving a first concentration of lithium salt less than 3.0 M (mole/L),but greater than 0.001M; and (B) removing a portion of the first liquidsolvent to obtain said quasi-solid electrolyte having a final lithiumsalt concentration higher than the first concentration so that theelectrolyte exhibits a vapor pressure less than 0.01 kPa when measuredat 20° C., a vapor pressure less than 60% of the vapor pressure of thefirst liquid solvent alone, a flash point at least 20 degrees Celsiushigher than the flash point of the first liquid solvent alone, a flashpoint higher than 150° C., or no detectable flash point.

Preferably, the first concentration of lithium salt is less than 2.0 Mand further preferably less than 1.0 M to improve the flowability (easeto flow) of the resulting liquid electrolyte to facilitate impregnationof the electrolyte into pores of a porous separator or an electrode(anode or cathode).

Preferably, the final lithium salt concentration is greater than 2.5 Mand/or greater than a molecular ratio of 0.2. More preferably, the finallithium salt concentration is greater than 3.5 M and/or greater than amolecular ratio of 0.3. Further preferably, the final lithium saltconcentration is greater than 5.0 M and/or greater than a molecularratio of 0.4. In certain embodiments, the final lithium saltconcentration is greater than 10 M and/or greater than a molecular ratioof 0.6. The lithium salt concentration can be effectively greater than20 M. Typically, the molecular ratio is from 0.3 to 0.99.

Preferably, the final lithium salt concentration is from 3.5 M to 15 Mand/or has a molecular ratio from 0.2 to 0.9.

Typically, the electrolyte has a lithium ion transference number greaterthan 0.4, more preferably greater than 0.6, and further preferablygreater than 0.7.

In the disclosed method, the first liquid solvent may be selected fromthe group consisting of 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME),tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE),2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylene carbonate (EC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), ethyl propionate, methyl propionate, propylene carbonate (PC),gamma-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA),propyl formate (PF), methyl formate (MF), toluene, xylene, methylacetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC),allyl ethyl carbonate (AEC), a hydrofloroether, a room-temperature ionicliquid solvent, and combinations thereof. Other solvents that arecapable of dissolving a lithium salt may also be used.

In someembodiments, the ionic liquid solvent has a cation selected fromtetraalkylammonium, di-, tri-, or tetra-alkylimidazolium,alkylpyridinium, dialkyl-pyrrolidinium, dialkylpiperidinium,tetraalkylphosphonium, trialkylsulfonium, or a combination thereof. Theionic liquid solvent may have an anion selected from BF₄ ⁻, B(CN)₄ ⁻,CH₃BF₃ ⁻, CH₂CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻,N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, or acombination thereof.

The lithium salt may be selected from lithium perchlorate (LiClO₄),lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄),lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate(LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂),lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate(LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethylsulfonylimide (LiBETI),lithium bis(trifluoromethanesulfonyl)imide, lithiumbis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI),an ionic liquid-based lithium salt, or a combination thereof.

In some embodiments, the first liquid solvent further comprises anadditive. The additive is different in composition than the first liquidsolvent and may be selected from Hydrofluoro ether (HFE). Trifluoropropylene carbonate (FPC), Methyl nonafluorobutyl ether (MFE),Fluoroethylene carbonate (FEC), Tris(trimethylsilyl)phosphite (TTSPi),Triallyl phosphate (TAP), Ethylene sulfate (DTD), 1,3-propane sultone(PS), Propene sultone (PES), Alkylsiloxane (Si—O), Alkyylsilane (Si—C),liquid oligomeric silaxane (—Si—O—Si—), Ttetraethylene glycoldimethylether (TEGDME), canola oil, or a combination thereof and saidadditive-to-said liquid solvent ratio in said mixture is from 1/95 to99/1 by weight.

In certain embodiments, the first liquid solvent further comprises asecond liquid solvent mixed with the first liquid solvent to dissolvethe lithium salt and the method further comprises partially or totallyremoving the second solvent after the lithium salt is dissolved.

The second liquid solvent may be preferably selected from acetone, analcohol (methanol, ethanol, propanol, etc.), acetonitrile, an ether-typesolvent, or a combination thereof. The ether-type solvent may beselected from 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME),tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE),2-ethoxyethyl ether (EEE), sulfone, or sulfolane

Also provided in the present disclosure is a method of producing aseparator/quasi-solid electrolyte layer for use in a lithium battery,the method comprising: (a) providing an ion-permeable or porousthin-film separator, wherein the separator has a thickness from 1 μm to500 μm; (b) dissolving a lithium salt in a first liquid solvent toobtain a flowable liquid mixture having a first concentration of lithiumsalt less than 3.0 M (mole/L), but greater than 0.001M; and (c) coatingor impregnating the thin-film separator with the flowable liquid mixtureto obtain a separator/electrolyte layer and removing a portion of thefirst liquid solvent to obtain a separator/quasi-solid electrolytehaving a final lithium salt concentration higher than the firstconcentration so that the electrolyte exhibits a vapor pressure lessthan 0.01 kPa when measured at 20° C., a vapor pressure less than 60% ofthe vapor pressure of said first liquid solvent alone, a flash point atleast 20 degrees Celsius higher than a flash point of the first liquidsolvent alone, a flash point higher than 150° C., or no detectable flashpoint.

In (c), the electrolyte may be combined with a porous film to form aseparator-electrolyte layer through co-lamination or coating.

Such a separator/electrolyte layer may be used in a rechargeable lithiumbattery, including various types of lithium-ion cells, lithium metalcells, lithium-sulfur cell, lithium-selenium cell, and lithium-air cell.This battery features a non-flammable, safe, and high-performingelectrolyte.

Preferably and typically, the final lithium salt concentration isgreater than 3.5 M and/or greater than a molecular ratio of 0.2. Morepreferably and typically, the final lithium salt concentration isgreater than 5.0 M and/or greater than a molecular ratio of 0.3. Quiteoften, the final lithium salt concentration is greater than 7.0 M and/orgreater than a molecular ratio of 0.4. In several cases, the finallithium salt concentration is greater than 8.5 M and/or greater than amolecular ratio of 0.5. We have also routinely achieved an ultra-highlithium salt concentration higher than 9-14 M, which have not beenconsidered possible or desirable by electrochemists and batterydesigners.

The invented electrolyte typically has a lithium ion transference numbergreater than 0.4, often greater than 0.6, and even greater than 0.7.

The first liquid solvent may be selected from the group consisting of1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycoldimethylether (TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME),diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE),sulfone, sulfolane, ethylene carbonate (EC), dimethyl carbonate (DMC),methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate,methyl propionate, propylene carbonate (PC), gamma-butyrolactone (γ-BL),acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methylformate (MF), toluene, xylene, methyl acetate (MA), fluoroethylenecarbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC), ahydrofloroether, a room-temperature ionic liquid solvent, andcombinations thereof.

The ionic liquid solvent may have a cation selected fromtetraalkylammonium, di-, tri-, or tetra-alkylimidazolium,alkylpyridinium, dialkyl-pyrrolidinium, dialkylpiperidinium,tetraalkylphosphonium, trialkylsulfonium, or a combination thereof. Thecorresponding anion for the ionic liquid solvent may be selected fromBF₄ ⁻, B(CN)₄ ⁻, CH₃BF₃ ⁻, CH₂CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻,n-C₄F₉BF₃ ⁻, PF₆ ⁻, CF₃CO₂, CF₃SO₃, N(SO₂CF₃)₂, N(COCF₃)(SO₂CF₃)N(SO₂F)₂ ⁻, N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻,F(HF)_(2.3) ⁻, or a combination thereof. The lithium salt used to makethe electrolyte may be selected from lithium perchlorate (LiClO₄),lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄),lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate(LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂),lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate(LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethylsulfonylimide (LiBETI),lithium bis(trifluoromethanesulfonyl)imide, lithiumbis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI),an ionic liquid-based lithium salt, or a combination thereof.

The electrolyte may contain a mixture of an organic liquid solvent andan ionic liquid solvent. Preferably, the organic liquid solvent-to-ionicliquid solvent ratio is greater than 1/1.

A particularly desired ionic liquid solvent is selected from a roomtemperature ionic liquid having a cation selected fromtetraalkylammonium, di-, tri-, or tetra-alkylimidazolium,alkylpyridinium, dialkyl-pyrrolidinium, dialkylpiperidinium,tetraalkylphosphonium, trialkylsulfonium, or a combination thereof. Thedesirable anion may be selected from BF₄ ⁻, B(CN)₄ ⁻, CH₃BF₃ ⁻, CH₂CHBF₃⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆ ⁻, CF₃CO₂ ⁻,CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻, N(CN)₂ ⁻, C(CN)₃⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, or a combinationthereof.

A preferred embodiment of the present disclosure is a roll-to-rollprocess wherein (a) entails continuously or intermittently feeding aporous thin-film separator from a feeder roller into acoating/impregnation zone and (c) entails collecting theseparator-electrolyte layer on a winding roller. Specifically, in theroll-to-roll process, (a) entails continuously or intermittently feedinga porous thin-film separator sheet from a feeder roller, (c) entailsdepositing the electrolyte (the flowable liquid mixture) onto one or twoprimary surfaces of the separator sheet or impregnating pores of theseparator sheets with the electrolyte and then partially removing theliquid solvent to form the separator-electrolyte layer, which isfollowed by collecting the separator-electrolyte layer on a windingroller. This impregnation operation may be advantageously accomplishedby feeding the porous separator sheet to get immersed into theelectrolyte solution in an electrolyte bath.

Alternatively, (c) may entail spraying and depositing the lithium saltand the first liquid solvent concurrently or sequentially onto one ortwo primary surfaces of the porous separator sheet to form theseparator-electrolyte layer. Further alternatively, a desired amount oflithium salt may be mixed and dissolved in the first solvent to form anelectrolyte solution (e.g. having initially a higher-than-desired amountof the first solvent). The operation (c) then further includes removingthe excess portion of the first liquid solvent to increase the lithiumsalt concentration.

We have surprisingly discovered that the flammability of any organicsolvent can be effectively suppressed provided that a sufficiently highamount of a lithium salt is added to and dissolved in this organicsolvent to form a solid-like or quasi-solid electrolyte. In general,such a quasi-solid electrolyte exhibits a vapor pressure less than 0.01kPa (when measured at 20° C.) and less than 0.1 kPa (when measured at100° C.). In many cases, the vapor molecules are practically too few tobe detected. The high solubility of the lithium salt in an otherwisehighly volatile solvent has effectively prevented the flammable gasmolecules from initiating a flame even at an extremely high temperature(e.g. using a torch, as demonstrated in FIG. 1). The flash point of thequasi-solid electrolyte is typically at least 20 degrees (often >50degrees) higher than the flash point of the neat organic solvent alone.In most of the cases, either the flash point is higher than 150° C. orno flash point can be detected. The electrolyte just would not catch onfire or get ignited. Any accidentally initiated flame does not sustainfor longer than a few seconds. This is a highly significant discovery,considering the notion that fire and explosion concern has been a majorimpediment to widespread acceptance of battery-powered electricvehicles. This new technology could potentially reshape the landscape ofEV industry. A mixture with no detectable flash point or no flash pointobserved does not ignite when exposed to a methane torch (methane gasburns at 1950° C.).

Another surprising element of the present disclosure is the notion thatwe are able to dissolve a high concentration of a lithium salt in anorganic solvent to form an electrolyte suitable for use in arechargeable lithium battery. This concentration is typically greaterthan a lithium salt molecular ratio >0.2, more typically >0.3, canbe >0.4, 0.5, 0.6, and even >0.7. In a more easily understood butscientifically not accurate manner, the concentration is typicallygreater than 3.5 M (mole/liter), more typically and preferably greaterthan 4 M, still more typically and preferably greater than 5 M, furthermore preferably and typically greater than 7 M, and most preferablygreater than 10 M. Generally, such a high concentration of lithium saltin a solvent has not been considered possible. Actually, such a highconcentration is considered undesirable.

Indeed, in general, it has not been possible to achieve such a highconcentration of lithium salt in an organic solvent used in a batteryelectrolyte. After an extensive and in-depth study, we came to discoverthat the apparent solubility of a lithium salt in a solvent could besignificantly increased if a lower concentration of lithium salt (highersolvent content) is implemented to make the resulting mixture morefluidy (more flowable) and then a portion of the liquid solvent isremoved after the electrolyte is coated onto separator surfaces orimpregnated into pores of the separator. Alternatively, a desirableoutcome may be achieved if (a) a highly volatile co-solvent is used toincrease the amount of lithium salt dissolved in the solvent mixturefirst and then (b) this volatile co-solvent is partially or totallyremoved once the dissolution procedure is completed and the electrolytesolution is coated onto a porous separator layer. The resultingelectrolyte is now in a solid-like state (having just a small amount ofliquid solvent remaining in the electrolyte), herein referred to as aquasi-solid electrolyte. Such a more rigid state enables the resultingseparator/electrolyte laminate to be readily combined with an anode anda cathode to make a battery cell. This cell has already had anelectrolyte and, as such, requires no subsequent injection of a liquidelectrolyte after the cell is made. In contrast, the conventionalprocess of making a lithium-ion cell entails making a dry cell, followedby injection of the liquid electrolyte into the empty cell.

Quite unexpectedly, the removal of this co-solvent typically did notlead to precipitation or crystallization of the lithium salt out of thesolution even though the solution would have been in a highlysupersaturated state. This novel and unique approach appears to haveproduced a material state wherein most of the solvent molecules areretained or captured by lithium salt ions that are not volatile. Hence,very few solvent molecules are able to escape into the vapor phase.Consequently, very few volatile gas molecules can be present to initiateor sustain a flame. This has not been taught or suggested as technicallypossible or viable in any previous report.

It may be noted that a good scientist in the field of chemistry ormaterials science would anticipate that such a high salt concentrationwould make the electrolyte behave like a solid with an extremely highviscosity and, hence, this electrolyte would not be amenable to fastdiffusion of lithium ions therein. Consequently, the scientist wouldexpect that a lithium battery containing such a solid-like electrolytecould not exhibit a high capacity at a high charge-discharge rate orunder a high current density condition (i.e. the battery would beexpected to have a poor rate capability). Contrary to theseexpectations, all the lithium cells containing such a quasi-solidelectrolyte deliver surprisingly high energy density and high powerdensity for a long cycle life. The quasi-solid electrolytes as hereindisclosed are conducive to facile lithium ion transport. This surprisingobservation is manifested by a high lithium ion transference number(TN), to be further explained in a later section of this specification.We have found that the quasi-solid electrolytes provide a TN greaterthan 0.4 (typically in the range from 0.4-0.8), in contrast to thetypical values of 0.1-0.2 in all lower concentration electrolytes (e.g.<2.0 M) used in all current Li-ion and Li—S cells.

When at least two solvents are used, one solvent is preferably morevolatile than the other. For instance, the more volatile solvent may beselected from ether-like or ether-based solvents, such as 1,3-dioxolane(DOL), 1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether(TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME), diethyleneglycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, andsulfolane. The more volatile liquid solvent may be selected fromacetone, an alcohol (methanol, ethanol, propanol, etc.), acetonitrile,an ether-type solvent, or a combination thereof. After processing, thismore volatile solvent can be more readily removed to increase theeffective lithium salt concentration.

A less volatile solvent (relative to the ether-type) may be selectedfrom ethylene carbonate (EC), dimethyl carbonate (DMC), methylethylcarbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methylpropionate, propylene carbonate (PC), gamma.-butyrolactone (γ-BL),acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methylformate (MF), toluene, xylene, methyl acetate (MA), fluoroethylenecarbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC),or a hydrofloroether. However, these solvents are significantly morevolatile than the ionic liquid solvents and, hence, can be used as aco-solvent in the presently invented process.

In a lithium metal secondary cell or a lithium-ion cell, the lithiumsalt may be selected from lithium perchlorate (LiClO₄), lithiumhexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithiumhexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate(LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂),lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate(LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethylsulfonylimide (LiBETI),lithium bis(trifluoromethanesulfonyl)imide, lithiumbis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI),an ionic liquid-based lithium salt, or a combination thereof.

The rechargeable lithium cell of the present disclosure featuring anon-flammable quasi-solid electrolyte is not limited to lithiummetal-sulfur cell or lithium-ion cell. This safe and high-performingelectrolyte can be used in any lithium metal secondary cell (lithiummetal-based anode coupled with any cathode active material) and anylithium-ion cell.

These and other advantages and features of the present disclosure willbecome more transparent with the description of the following best modepractice and illustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Photos showing the results of a flammability test conducted forvarious electrolytes with different lithium salt concentrations.

FIG. 2 Vapor pressure ratio data (p_(s)/p=vapor pressure ofsolution/vapor pressure of solvent alone) as a function of the lithiumsalt molecular ratio x (LiTFSI/DOL), along with the theoreticalpredictions based on the classic Raoult's Law.

FIG. 3 Vapor pressure ratio data (p_(s)/p=vapor pressure ofsolution/vapor pressure of solvent alone) as a function of the lithiumsalt molecular ratio x (LiTFSI/DME), along with the theoreticalpredictions based on classic Raoult's Law.

FIG. 4 Vapor pressure ratio data (p_(s)/p=vapor pressure ofsolution/vapor pressure of solvent alone) as a function of the lithiumsalt molecular ratio x (LiPF₆/DOL), along with the theoreticalpredictions based on classic Raoult's Law.

FIG. 5 Vapor pressure ratio data (p_(s)/p=vapor pressure ofsolution/vapor pressure of solvent alone) as a function of the lithiumsalt molecular ratio x (LiTFSI/DOL, LiTFSI/DME, LiPF₆/DOL), along withthe theoretical predictions based on classic Raoult's Law.

FIG. 6 The Li⁺ ion transference numbers of electrolytes (e.g. LiTFSIsalt/(DOL+DME) solvents) in relation to the lithium salt molecular ratiox.

FIG. 7 The Li⁺ ion transference numbers of electrolytes (e.g. LiTFSIsalt/(EMImTFSI+DOL) solvents) in relation to the lithium salt molecularratio x.

FIG. 8 The Li⁺ ion transference numbers of electrolytes (e.g. LiTFSIsalt/(EMImTFSI+DME) solvents) in relation to the lithium salt molecularratio x.

FIG. 9 The Li⁺ ion transference numbers in various electrolytes (as inFIG. 6-FIG. 8) in relation to the lithium salt molecular ratio x.

FIG. 10 Cycling performance (discharge specific capacity) of a Limetal-sulfur cell containing a low-concentration electrolyte (x=0.06) ofLi salt in an organic solvent) and that of a Li metal-sulfur cellcontaining a high-concentration organic electrolyte (x=0.35). Initialspecific capacity of the cathode was 1488 mAh/g (based on the S weight).

FIG. 11 Ragone plots (cell power density vs. cell energy density) ofthree Li metal-sulfur cells each having an exfoliated graphiteworm-sulfur cathode, but the lithium salt concentrations being x=0.07,0.24, and 0.35, respectively.

FIG. 12(A) Schematic of roll-to-roll electrolyte-separator productionsystem, having a dispenser 14 supplying pre-mixed lithium salt-solventelectrolyte 16;

FIG. 12(B) having an electrolyte bath 34 for impregnating the porousseparator film 32;

FIG. 12(C) having separate salt and solvent dispensers 54, 56.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present disclosure provides a method of producing a safe andhigh-performing electrolyte and electrolyte-separator layer for use in arechargeable lithium battery, which can be any of various types oflithium-ion cells or lithium metal cells. The electrolyte orelectrolyte-separator layer may also be used for a primary(non-rechargeable) lithium battery. A high degree of safety is impartedto this battery by a novel and unique electrolyte that is essentiallynon-flammable and would not initiate a fire or sustain a fire, even ifaccidentally initiated, and, hence, would not pose explosion danger.This disclosure has solved the very most critical issue that has plaguedthe lithium-metal and lithium-ion industries for more than two decades.

In certain embodiments, the present disclosure provides a method ofproducing a non-flammable quasi-solid electrolyte for a lithium battery,the method comprising (A) dissolving a lithium salt in a first liquidsolvent to obtain a mixture having a first concentration of lithium saltless than 3.0 M (mole/L), but greater than 0.001M; and (B) removing aportion of the first liquid solvent to obtain said quasi-solidelectrolyte having a final lithium salt concentration higher than thefirst concentration so that the electrolyte exhibits a vapor pressureless than 0.01 kPa when measured at 20° C., a vapor pressure less than60% of the vapor pressure of the first liquid solvent alone, a flashpoint at least 20 degrees Celsius higher than the flash point of thefirst liquid solvent alone, a flash point higher than 150° C., or nodetectable flash point. A mixture with no detectable flash point or noflash point observed does not ignite when exposed to a methane torch(methane gas burns at 1950° C.). FIG. 1 demonstrates a flame test wheremixtures of LiTFSI/DME+DOL are exposed to an external flame source. FIG.1 shows that as the concentration of the mixture increases, the mixturebecomes less able to sustain a flame introduced by an external flamesource. In this case, a butane torch (not shown in the figure) providesan external flame, some of which are seen in the figures. The mixture oflower concentration (0.077 ratio) shows a flash point condition or acondition in which the mixture contributes to the flame. As theconcentration is raised, the mixture contributes less to the externalflame. The 0.23 ratio mixture shows a smaller flame, as the mixturecontributes less. The mixtures with ratios of 0.3, 0.334, and 0.37provide significantly less, if any, contribution to the external flame.

In certain embodiments, the present disclosure also provides a method ofproducing a separator/quasi-solid electrolyte layer for use in a lithiumbattery, the method comprising: (a) providing an ion-permeable or porousthin-film separator, wherein the separator has a thickness from 1 μm to500 μm; (b) dissolving a lithium salt in a first liquid solvent toobtain a flowable liquid mixture having a first concentration of lithiumsalt less than 3.0 M (mole/L), but greater than 0.001M; and (c) coatingor impregnating the thin-film separator with the flowable liquid mixtureto obtain a separator/electrolyte layer and removing a portion of thefirst liquid solvent to obtain a separator/quasi-solid electrolytehaving a final lithium salt concentration higher than the firstconcentration so that the electrolyte exhibits a vapor pressure lessthan 0.01 kPa when measured at 20° C., a vapor pressure less than 60% ofthe vapor pressure of the first liquid solvent alone (without anyadditive or salt), a flash point at least 20 degrees Celsius higher thana flash point of the first liquid solvent alone, a flash point higherthan 150° C., or no detectable flash point. Such an electrolyte issurprisingly safe, non-flammable and not explosion-prone.

The porous thin-film separator may be selected from a porous polymerfilm, a porous mat, fabric, or paper made of polymer or glass fibers, ora combination thereof. The mat of polymer nano-fibers produced byelectro-spinning is particularly useful for supporting the presentlyinvented quasi-solid electrolyte.

By implementing this inventive non-flammable electrolyte-separatorsystem, the resulting lithium battery typically comprises a cathodehaving a cathode active material and/or a conductive cathode-supportingstructure, an anode having an anode active material and/or a conductivesupporting nano-structure, a separator electronically separating theanode and the cathode, an organic solvent-based highly concentratedelectrolyte in contact with the cathode active material (or the cathodeconductive supporting structure for a Li-air cell) and the anode activematerial. The electrolyte contains a lithium salt dissolved in a firstorganic liquid solvent with a lithium salt molecular ratio sufficientlyhigh so that the electrolyte exhibits a vapor pressure less than 0.01kPa or less than 0.6 (60%) of the vapor pressure of the solvent alone(when measured at 20° C.), a flash point at least 20 degrees Celsiushigher than a flash point of the first organic liquid solvent alone(when no lithium salt is present), a flash point higher than 150° C., orno detectable flash point at all.

Most surprising and of tremendous scientific and technologicalsignificance is our discovery that the flammability of any volatileorganic solvent can be effectively suppressed provided that asufficiently high amount of a lithium salt is added to and dissolved inthis organic solvent to form a solid-like or quasi-solid electrolyte. Ingeneral, such a quasi-solid electrolyte exhibits a vapor pressure lessthan 0.01 kPa and often less than 0.001 kPa (when measured at 20° C.)and less than 0.1 kPa and often less than 0.01 kPa (when measured at100° C.). (The vapor pressures of the corresponding neat solvent,without any lithium salt dissolved therein, are typically significantlyhigher.) In many cases, the vapor molecules are practically too few tobe detected.

A highly significant observation is that the high solubility of thelithium salt in an otherwise highly volatile solvent (a large molecularratio or molar fraction of lithium salt, typically >0.2, moretypically >0.3, and often >0.4 or even >0.5) has dramatically curtailedthe amount of volatile solvent molecules that can escape into the vaporphase in a thermodynamic equilibrium condition. In many cases, this haseffectively prevented the flammable gas molecules from initiating aflame even at an extremely high temperature (e.g. using a torch, asdemonstrated in FIG. 1). The flash point of the quasi-solid electrolyteis typically at least 20 degrees (often >50 degrees) higher than theflash point of the neat organic solvent alone. In most of the cases,either the flash point is higher than 150° C. or no flash point can bedetected. The electrolyte just would not catch on fire. Furthermore, anyaccidentally initiated flame does not sustain for longer than 3 seconds.This is a highly significant discovery, considering the notion that fireand explosion concern has been a major impediment to widespreadacceptance of battery-powered electric vehicles. This new technologycould significantly impact the emergence of a vibrant EV industry.

A preferred embodiment of the present disclosure is a roll-to-rollprocess. As illustrated in FIG. 12(A), the process can begin withcontinuously or intermittently feeding a porous thin-film separator 12from a feeder roller 10 into an electrolyte mixing zone, wherein ancontrolled amount of the solution electrolyte 16 of a lithium saltdissolved in a first solvent is supplied from a dispenser 14 and iscoated onto the top and/or bottom surfaces of the porous separator. Asthe electrolyte-coated separator layer is moved further to the right,there is a solvent removing provision (e.g. a ventilation systemequipped with a vacuum pump) to remove a desired amount of the liquidsolvent. An optional set of rollers 18, 19 may be used to work theelectrolyte into the pores of the separator and enable the electrolytelayer(s) to be well adhered to the separator surface(s) with a uniformcoating thickness. The resulting electrolyte-separator layer 20 isfurther moved into an optional solvent removal zone 22, wherein adesired amount of volatile solvent is removed (e.g. vaporized) toeffectively further increase the lithium salt concentration. Thissolvent removal zone is equipped with heating, ventilation, and/orvacuum pumping provisions to facilitate solvent vaporization. This isfollowed by collecting the separator-electrolyte layer on a windingroller 24. Optionally, the separator-electrolyte layer may be combinedwith a backing layer (releasing paper) prior to being collected into aroll.

Alternatively, as schematically shown in FIG. 12(B), the roll-to-rollprocess begins with (a) that entails continuously or intermittentlyfeeding a porous thin-film separator sheet 32 from a feeder roller 30into an impregnation bath 34. As (c) of the process, the separator sheetpicks up a desired amount of the electrolyte on its two surfaces and theliquid electrolyte also impregnate the pores of the separator sheets,forming a separator-electrolyte layer 38, which is then moved through asolvent removal zone 40. (c) further entails collecting theseparator-electrolyte layer on a winding roller 44. The gap between thepair 36 of consolidating rollers regulates the amount of electrolytesolution adhered onto the porous separator film. Again, an optionalevaporation zone is implemented in front of the winder roller.

Further alternatively, as illustrated in FIG. 12(C), (a) includesfeeding a porous separator film 52 from a feeder roller 50. (c) mayentail spraying and depositing the lithium salt 53 b and the firstliquid solvent 53 a concurrently or sequentially to form a lithiumsalt-solvent mixture 57 onto one or two primary surfaces of the porousseparator sheet. The porous separator sheet with the salt-solventmixture supported thereon moves through a pair of consolidating rollers58, 59 to form the separator-electrolyte layer 60. The mixture initiallycan have a higher-than-desired amount of the first solvent. (c) mayinclude removing the excess portion of the first liquid solvent beforeand/or after passing through the roller gap (e.g. by passing through asolvent removal zone 62) to increase the lithium salt concentration in asolvent removal zone. The separator-electrolyte layer, along with anoptional release paper (backing layer), are then collected on a windingroller.

It may be noted that the quasi-solid electrolyte-coated porousseparator, as a separate product, has several advantages:

-   -   (a) This separator-electrolyte layer can be laminated with an        anode layer (e.g. graphite particle-coated Cu foil) and a        cathode layer (e.g. lithium iron phosphate particle-coated Al        foil) to form a battery laminate using a highly automated        lamination machine for mass production of battery cells. This        separator-electrolyte product obviates the need to undergo        electrolyte injection into a cell after the cell is produced.        This conventional practice of post-cell electrolyte injection        has been a bottle neck in the entire battery production process.        This conventional practice requires the use of additional        equipment that must be operated in a moisture-free environment        and can be labor intensive.    -   (b) The need to undergo post-cell electrolyte injection in the        conventional lithium-ion battery production also implies that        the cell cannot be highly compacted during the lamination        process. The presently invented quasi-solid        electrolyte-separator product enables direct lamination of        components with a high degree of compaction, in a highly        automated manner. The resulting battery cells have a        significantly higher energy density (wh/L). This is a critically        important feature when the battery space is premium (e.g. for        battery to be included in a slim smart phone where thickness is        important).    -   (c) The quasi-solid electrolyte layer leads to a more uniform        distribution of electrolyte in a battery cell.    -   (d) The quasi-solid electrolyte layer makes it possible for the        resulting battery cell to be free from liquid electrolyte        leakage issues.

Not wishing to be bound by theory, but we would like to offer sometheoretical aspects of the presently invented quasi-solid electrolytes.From the perspective of fundamental chemistry principles, addition ofsolute molecules to a liquid elevates the boiling temperature of theliquid and reduces its vapor pressure and freezing temperature. Thesephenomena, as well as osmosis, depend only on the solute concentrationand not on its type, and are called colligative properties of solutions.The original Raoult's law provides the relationship between the ratio ofthe vapor pressure (p_(s)) of a solution to the vapor pressure (p) ofthe pure liquid and the molar fraction of the solute (x):

p _(s) /p=e ^(−x)  Eq.(1a)

For a dilute solution, x<<1 and, hence, e^(−x≈)1−x. Thus, for thespecial cases of low solute molar fractions, one obtains a more familiarform of Raoult's law:

p _(s) /p=1−x  Eq.(1b)

In order to determine if the classic Raoult's law can be used to predictthe vapor pressures of highly concentrated electrolytes, we proceeded toinvestigate a broad array of lithium salt/organic solvent combinations.Some of the examples of our research results are summarized in FIG.2-FIG. 5, where the experimental pip values are plotted as a function ofthe molecular ratio (molar fraction, x) for several salt/solventcombinations. Also plotted for comparison purpose is a curve based onthe classic Raoult's law, Eq. (1a). It is clear that, for all types ofelectrolytes, the pip values follow the Raoult's law prediction untilthe molar fraction x reaches approximately 0.2, beyond which the vaporpressure rapidly drops to essentially zero (barely detectable). When avapor pressure is lower than a threshold, no flame would be initiated,and we are proud to state that the present disclosure provides aplatform materials chemistry approach to effectively suppress theinitiation of flame.

Although deviations from Raoult's law are not uncommon in science, butthis type of curve for the pip values has never been observed for anybinary solution systems. In particular, there has been no study reportedon the vapor pressure of ultra-high concentration battery electrolytes(with a high molecular fraction, e.g. >0.2 or >0.3) for safetyconsiderations. This is truly unexpected and of technological andscientific significance.

Another surprising element of the present disclosure is the notion thatwe are able to dissolve a high concentration of a lithium salt in justabout every type of commonly used battery-grade organic solvent to forma quasi-solid electrolyte suitable for use in a rechargeable lithiumbattery. Expressed in a more easily recognizable term, thisconcentration is typically greater than 3.5 M (mole/liter), moretypically and preferably greater than 4 M, still more typically andpreferably greater than 5 M, further more preferably greater than 7 M,and most preferably greater than 10 M. Such a high concentration oflithium salt in a solvent has not been generally considered possible.However, one must understand that the vapor pressure of a solutioncannot be predicted directly and straightforwardly from theconcentration value in terms of M (mole/liter). Instead, for a lithiumsalt, the molecular ratio x in Raoult's law is the sum of the molarfractions of positive ions and negative ions, which is proportional tothe degree of dissociation of a lithium salt in a particular solvent ata given temperature. The mole/liter concentrations do not provideadequate information to enable prediction of vapor pressures.

In general, it has not been possible to achieve such a highconcentration of lithium salt (e.g., x=0.3-0.7) in an organic solventused in a battery electrolyte. After an extensive and in-depth study, wecame to further discover that the apparent solubility of a lithium saltin a particular solvent could be significantly increased if (a) a highlyvolatile co-solvent is used to increase the amount of lithium saltdissolved in the solvent mixture first and then (b) this volatileco-solvent is partially or totally removed once the dissolutionprocedure is completed. Quite unexpectedly, the removal of thisco-solvent typically did not lead to precipitation or crystallization ofthe lithium salt out of the solution even though the solution would havebeen in a highly supersaturated state. This novel and unique approachappears to have produced a material state wherein most of the solventmolecules are captured or held in place by lithium salt ions that arenot volatile (actually the lithium salt being like a solid). Therefore,very few volatile solvent molecules are able to escape into the vaporphase and, hence, very few “flammable” gas molecules are present to helpinitiate or sustain a flame. This has not been suggested as technicallypossible or viable in the prior art.

Furthermore, a skilled artisan in the field of chemistry or materialsscience would have anticipated that such a high salt concentrationshould make the electrolyte behave like a solid with an extremely highviscosity and, hence, this electrolyte should not be amenable to fastdiffusion of lithium ions therein. Consequently, the artisan would haveexpected that a lithium battery containing such a solid-like electrolytewould not and could not exhibit a high capacity at a highcharge-discharge rate or under a high current density condition (i.e.the battery should have a poor rate capability). Contrary to theseexpectations by a person of ordinary skills or even exceptional skillsin the art, all the lithium cells containing such a quasi-solidelectrolyte deliver high energy density and high power density for along cycle life. It appears that the quasi-solid electrolytes as hereininvented and disclosed are conducive to facile lithium ion transport.This surprising observation is related to a high lithium iontransference number (TN), to be further explained in a later section ofthis specification. We have found that the quasi-solid electrolytesprovides a TN greater than 0.4 (typically in the range from 0.4-0.8), incontrast to the typical values of 0.1-0.2 in all lower concentrationelectrolytes (e.g. <2.0 M) used in all current Li-ion and Li—S cells.

As indicated in FIG. 6-FIG. 9, the Li⁺ ion transference number in lowsalt concentration electrolytes decreases with increasing concentrationfrom x=0 to x=0.2-0.35. However, beyond molecular ratios of x=0.2-0.35,the transference number increases with increasing salt concentration,indicating a fundamental change in the Li⁺ ion transport mechanism. Notwishing to be bound by theory, but we would like to offer the followingscientifically plausible explanations: When Li⁺ ions travel in a lowsalt concentration electrolyte (e.g. x<0.2), each Li⁺ ion drags one ormore solvating anions along with it. The coordinated migration of such acluster of charged species can be further impeded if the fluid viscosityis increased (i.e. if more salt is added to the solvent).

Fortunately, when an ultra-high concentration of lithium salt (e.g.,with x>0.2) is present, Li⁺ ions could significantly out-number theavailable solvating anions or solvent molecules that otherwise couldcluster the lithium ions, forming multi-ion complex species that slowdown the diffusion process of Li⁺ ions. Presumably, the high viscosityin a high-concentration electrolyte has a more significant effect oncurtailing the mobility of generally larger anions than it does tosmaller Li⁺ ions. This effect and the high Li⁺ ion concentration make itpossible to have more “free Li⁺ ions” (those acting alone without beingclustered), thereby providing a high Li⁺ transference number (hence, afacile Li⁺ transport). In other words, the lithium ion transportmechanism changes from a multi-ion complex-dominating one (with a largerhydrodynamic radius) to single ion-dominating one (with a smallerhydrodynamic radius) having a large number of available free Li⁺ ions.This observation has further asserted that Li⁺ ions can operate onquasi-solid electrolytes without compromising the rate capability of aLi—S cell. Yet, these highly concentrated electrolytes are non-flammableand safe. These combined features and advantages for batteryapplications have never been taught or even slightly hinted in anyprevious report. Theoretical aspects of the ion transference number ofquasi-solid electrolytes are now presented below:

In selecting an electrolyte system for a battery, the ionic conductivityof lithium ions is an important factor to consider. The ionicconductivity of Li⁺ ions in an organic liquid-based electrolyte is onthe order of 10⁻³-10⁻² S/cm and that in a solid state electrolyte istypically in the range from 10⁻⁴-10⁻⁶ S/cm. Due to the low ionicconductivity, solid-state electrolytes have not been used to anysignificant extent in any battery system. This is a pity sincesolid-state electrolyte is resistant to dendrite penetration in alithium metal secondary cell and does not allow for dissolution oflithium polysulfide in a Li—S cell. The charge-discharge capacities ofLi—S cells with a solid electrolyte are extremely low, typically 1-2orders of magnitude lower than the theoretical capacity of sulfur. Incontrast, the ionic conductivity of our quasi-solid electrolytes istypically in the range from 10⁻⁴-8×10⁻³ S/cm, sufficient for use in arechargeable battery.

However, the overall ionic conductivity is not the only importanttransport parameter of a battery electrolyte. The individualtransference numbers of cations and anions are also important. Forinstance, when viscous liquids are used as electrolytes in lithiumbatteries high transference numbers of Li⁺ ions in the electrolyte areneeded.

The ion transport and diffusion in a liquid electrolyte consisting ofonly one type of cation (i.e. Li⁺) and one type of anion, plus a liquidsolvent or a mixture of two liquid solvents, may be studied by means ofAC impedance spectroscopy and pulsed field gradient NMR techniques. TheAC impedance provides information about the overall ionic conductivity,and NMR allows for the determination of the individual self-diffusioncoefficients of cations and anions. Generally, the self-diffusioncoefficients of the cations are slightly higher than those of theanions. The Haven ratio calculated from the diffusion coefficients andthe overall ionic conductivity is typically in the range from 1.3 to 2,indicating that transport of ion pairs or ion complexes (e.g. clustersof Lit solvating molecules) is an important feature in electrolytescontaining a low salt concentration.

The situation becomes more complicated when either two different lithiumsalts or one ionic liquid (as a lithium salt or liquid solvent) is addedto the electrolyte, resulting in a solution having at least 3 or 4 typesof ions. In this case, as an example, it is advantageous to use alithium salt containing the same anion as in the solvating ionic liquid,since the amount of dissolvable lithium salt is higher than in a mixturewith dissimilar anions. Thus, the next logical question to ask iswhether it is possible to improve the Li⁺ transference number bydissolving more lithium salt in liquid solvent.

The relation between the overall ionic conductivity of a three-ionliquid mixture, a_(dc), and the individual diffusion coefficients of theions, Di, may be given by the Nernst-Einstein equation:

σ_(dc)=(e ² /k _(B) TH _(R))[(N _(Li) ⁺)(D _(Li) ⁺)+(N _(A) ⁺)(D _(A)⁺)+(N _(B) ⁻)(D _(B) ⁻)]  Eq. (2)

Here, e and k_(B) denote the elementary charge and Boltzmann's constant,respectively, while N_(i) are the number densities of individual ions.The Haven ratio, H_(R), accounts for cross correlations between themovements of different types of ions.

Simple ionic liquids with only one type of cation and anion arecharacterized by Haven ratios being typically in the range from 1.3 to2.0. A Haven ratio larger than unity indicates that ions of dissimilarcharges move preferentially into the same direction (i.e. ions transportin pairs or clusters). Evidence for such ion pairs can be found usingRaman spectra of various electrolytes. The values for the Haven ratiosin the three-ion mixtures are in the range from 1.6 to 2.0. The slightlyhigher H_(R) values as compared to the electrolytes with x=0 indicatethat pair formation is more prominent in the mixtures.

For the same mixtures, the overall ionic conductivity of the mixturesdecreases with increasing lithium salt content x. This conductivity dropis directly related to a drop of the individual self-diffusioncoefficients of all ions. Furthermore, studies on different mixtures ofionic liquids with lithium salts have shown that the viscosity increaseswith increasing lithium salt content x. These findings suggest that theaddition of lithium salt leads to stronger ionic bonds in the liquidmixture, which slow down the liquid dynamics. This is possibly due tothe Coulomb interaction between the small lithium ions and the anionsbeing stronger than the Coulomb interactions between the larger organiccations and the anions. Thus, the decrease of the ionic conductivitywith increasing lithium salt content x is not due to a decreasing numberdensity of mobile ions, but to a decreasing mobility of the ions.

In order to analyze the individual contributions of the cations andanions to the overall ionic conductivity of the mixtures, one may definethe apparent transference numbers t, by:

t _(i) =N _(i) Di/(ΣN _(i) Di)  Eq.(3)

As an example, in a mixture of N-butyl-N-methyl-pyrrolidiniumbis(trifluoromethanesulfonyl) imide (BMP-TFSI) and lithiumbis(trifluoromethanesulfonyl)imide (Li-TFSI), containing Li⁺, BMP⁺, andTFSI⁻ ions, the apparent lithium transference number t_(Li) increaseswith increasing Li-TFSI content; at x=0.377, t_(Li)=0.132 (vs.t_(Li)<0.1 at x<0.2), D_(Li)≈0.8 D_(TFSI), and D_(BMP)≈1.6D_(TFSI). Themain reason for the higher apparent lithium transference number in themixture is the higher number density of lithium ions.

In order to further enhance the lithium transference number in suchmixtures, the number density and/or the diffusion coefficient of thelithium ions have to be further increased relative to the other ions. Afurther increase of the Li⁺ ion number density is generally believed tobe very challenging since the mixtures tend to undergo saltcrystallization or precipitation at high Li salt contents. The presentdisclosure has overcome this challenge. We have surprisingly observedthat the addition of a very small proportion of a highly volatileorganic liquid (e.g. an ether-based solvent) can significantly increasethe solubility limit of some Li salt in a highly viscous organic liquid(e.g. VC) or an ionic liquid (e.g. typically from x<0.2 to x>0.3-0.6, orfrom typically 1-2 M to >5 M). This can be achieved with an ionic liquid(or viscous organic liquid)-to-volatile organic solvent ratio as high as10:1, hence, keeping the volatile solvent content to a bare minimum andminimizing the potential flammability of the electrolyte.

The diffusion coefficients of the ions, as measured in the pulsed fieldgradient NMR (PFG-NMR) experiments, depend on the effective radius ofthe diffusing entities. Due to the strong interactions between Li⁺ ionsand TFSI⁻ ions, Li⁺ ions can form [Li(TFSI)_(n+1)]^(n−) complexes.Coordination numbers up to n+1=4 have been reported in open literature.The coordination number determines the effective hydrodynamic radius ofthe complex and thus the diffusion coefficient in the liquid mixture.The Stokes-Einstein equation, Di=k_(B)T/(cπηr_(i)), may be used tocalculate the effective hydrodynamic radius of a diffusing entity, ri,from its diffusion coefficient Di. The constant c varies between 4 and6, depending on the shape of the diffusing entity. A comparison of theeffective hydrodynamic radii of cations and anions in ionic liquids withtheir van der Waals radii reveals that the c values for cations aregenerally lower than for anions. In the case of EMI-TFSI/Li-TFSImixtures, hydrodynamic radii for Li are in the range from 0.7-0.9 nm.This is approximately the van der Waals radius of [Li(TFSI)₂]⁻ and[Li(TFSI)₃]²⁻ complexes. In the case of the BMP-TFSI/Li-TFSI mixturewith x=0.377, the effective hydrodynamic radius of the diffusing lithiumcomplex is r_(Li)=(D_(BMP)/D_(Li))r_(BMP)≈1.1 nm, under the assumptionthat r_(BMP)≈0.55 nm and that the c values for BMP⁺ and for thediffusing Li complex are identical. This value for r_(Li) suggests thatthe lithium coordination number in the diffusing complex is at least 2in the mixtures containing a low salt concentration.

Since the number of TFSI⁻ ions is not high enough to form a significantamount of [Li(TFSI)₃]²⁻ complexes, most lithium ions should be diffusingas [Li(TFSI)₂]⁻ complexes. If, on the other hand, higher Li saltconcentrations are achieved without crystallization (e.g. in ourquasi-solid electrolytes), then the mixtures should contain aconsiderable amount of neutral [Li(TFSI)] complexes, which are smaller(r[_(Li(TFsi)])≈0.4 nm) and should have higher diffusivities. Thus, ahigher salt concentration would not only enhance the number density oflithium ions but should also lead to higher diffusion coefficients ofthe diffusing lithium complexes relative to the organic cations. Theabove analysis is applicable to electrolytes containing either organicliquid solvents or ionic liquid solvents. In all cases, when the lithiumsalt concentrations are higher than a threshold, there will be anincreasing number of free or un-clustered Li⁺ ions to move between theanode and the cathode when the concentration is further increased,providing adequate amount of Li⁺ ions required forintercalation/deintercalation or chemical reactions at the cathode andthe anode.

In addition to the non-flammability and high lithium ion transferencenumbers as discussed above, there are several additional benefitsassociated with using the presently invented quasi-solid electrolytes.As one example, the quasi-solid electrolyte can significantly enhancecyclic and safety performance of rechargeable lithium batteries througheffective suppression of lithium dendrite growth. It is generallyaccepted that dendrites start to grow in the non-aqueous liquidelectrolyte when the anion is depleted in the vicinity of the electrodewhere plating occurs. In the ultrahigh concentration electrolyte, thereis a mass of anions to keep the balance of cations (Li⁺) and anions nearmetallic lithium anode. Further, the space charge created by aniondepletion is minimal, which is not conducive to dendrite growth.Furthermore, due to both ultrahigh lithium salt concentration and highlithium-ion transference number, the quasi-solid electrolyte provides alarge amount of available lithium-ion flux and raises the lithium ionicmass transfer rate between the electrolyte and the lithium electrode,thereby enhancing the lithium deposition uniformity and dissolutionduring charge/discharge processes. Additionally, the local highviscosity induced by a high concentration will increase the pressurefrom the electrolyte to inhibit dendrite growth, potentially resultingin a more uniform deposition on the surface of the anode. The highviscosity could also limit anion convection near the deposition area,promoting more uniform deposition of Li ions. These reasons, separatelyor in combination, are believed to be responsible for the notion that nodendrite-like feature has been observed with any of the large number ofrechargeable lithium cells that we have investigated thus far.

As another benefit example, this electrolyte is capable of inhibitinglithium polysulfide dissolution at the cathode of a Li—S cell, thusovercoming the polysulfide shuttle phenomenon and allowing the cellcapacity not to decay significantly with time. Consequently, a coulombicefficiency nearing 100% along with long cycle life has been achieved.The solubility of lithium polysulfide (ζ) is affected by theconcentration of lithium ions already present in the electrolyte by thecommon ion effect. The solubility product (K_(sp)) of lithiumpolysulfide may be written as:

Li ₂ S _(n)↔2Li ⁺ +S _(n) ²⁻ ;K _(sp)=[Li ⁺]²[S _(n) ²⁻]=4ξ_(o)³;ξ_(o)=(K _(sp)/4)^(1/3)  (Eq.4),

where ξ_(o) represents the solubility of lithium polysulfide when nolithium ion is present in the solvent. If the concentration of thelithium salt in the electrolyte (C) is significantly larger than thesolubility of polysulfide, the solubility of polysulfide in theelectrolyte containing the concentrated lithium salt can be expressedas:

ξ/ξ_(o)=(2ξ_(o) /C)²  (Eq.5).

Therefore, when a concentrated electrolyte is used, the solubility oflithium polysulfide will be reduced significantly.

The presently invented quasi-solid electrolyte and electrolyte-separatorlayer can be used in a rechargeable lithium cell selected from a lithiummetal secondary cell, a lithium-ion cell, a lithium-sulfur cell, alithium-ion sulfur cell, or a lithium-air cell. The rechargeable lithiumcell comprises a cathode having a cathode active material, an anodehaving an anode active material, a porous separator separating the anodeand the cathode, a non-flammable quasi-solid electrolyte in contact withthe cathode and the anode, wherein the electrolyte contains a lithiumsalt dissolved in a first organic liquid solvent with a concentrationsufficiently high so that the electrolyte exhibits a vapor pressure lessthan 0.01 kPa when measured at 20° C., a flash point at least 20 degreesCelsius higher than a flash point of said first organic liquid solventalone, a flash point higher than 150° C., or no flash point, wherein thelithium salt concentration x is greater than 0.2. The rechargeablelithium cell preferably contains a quasi-solid electrolyte having alithium ion transference number greater than 0.4, preferably andtypically greater than 0.6, and most preferably and typically greaterthan 0.7.

The first liquid solvent may be selected from the group consisting of1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycoldimethylether (TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME),diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE),sulfone, sulfolane, ethylene carbonate (EC), dimethyl carbonate (DMC),methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate,methyl propionate, propylene carbonate (PC), gamma.-butyrolactone(γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF),methyl formate (MF), toluene, xylene, methyl acetate (MA),fluoroethylene carbonate (FEC), vinylene carbonate (VC), allyl ethylcarbonate (AEC), a hydrofloroether (e.g. methyl perfluorobutyl ether,MFE, or ethyl perfluorobutyl ether, EFE), and combinations thereof.

The lithium salt may be selected from lithium perchlorate (LiClO₄),lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄),lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate(LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂),lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate(LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethylsulfonylimide (LiBETI),lithium bis(trifluoromethanesulfonyl)imide, lithiumbis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI),an ionic liquid-based lithium salt, or a combination thereof.

The ionic liquid is composed of ions only. Ionic liquids are low meltingtemperature salts that are in a molten or liquid state when above adesired temperature. For instance, an ionic salt is considered as anionic liquid if its melting point is below 100° C. If the meltingtemperature is equal to or lower than room temperature (25° C.), thesalt is referred to as a room temperature ionic liquid (RTIL). TheIL-based lithium salts are characterized by weak interactions, due tothe combination of a large cation and a charge-delocalized anion. Thisresults in a low tendency to crystallize due to flexibility (anion) andasymmetry (cation).

Some ILs may be used as a co-solvent (not as a salt) to work with thefirst organic solvent of the present disclosure. A well-known ionicliquid is formed by the combination of a 1-ethyl-3-methyl-imidazolium(EMI) cation and an N,N-bis(trifluoromethane)sulphonamide (TFSI) anion.This combination gives a fluid with an ionic conductivity comparable tomany organic electrolyte solutions, a low decomposition propensity andlow vapor pressure up to ˜300-400° C. This implies a generally lowvolatility and non-flammability and, hence, a much safer electrolytesolvent for batteries.

Ionic liquids are basically composed of organic or inorganic ions thatcome in an unlimited number of structural variations owing to thepreparation ease of a large variety of their components. Thus, variouskinds of salts can be used to design the ionic liquid that has thedesired properties for a given application. These include, among others,imidazolium, pyrrolidinium and quaternary ammonium salts as cations andbis(trifluoromethanesulphonyl) imide, bis(fluorosulphonyl)imide andhexafluorophosphate as anions. Useful ionic liquid-based lithium salts(not solvent) may be composed of lithium ions as the cation andbis(trifluoromethanesulphonyl)imide, bis(fluorosulphonyl)imide andhexafluorophosphate as anions. For instance, lithiumtrifluoromethanesulfonimide (LiTFSI) is a particularly useful lithiumsalt.

Based on their compositions, ionic liquids come in different classesthat include three basic types: aprotic, protic and zwitterionic types,each one suitable for a specific application. Common cations of roomtemperature ionic liquids (RTILs) include, but are not limited to,tetraalkylammonium, di, tri, and tetra-alkylimidazolium,alkylpyridinium, dialkyl-pyrrolidinium, dialkylpiperidinium,tetraalkylphosphonium, and trialkylsulfonium. Common anions of RTILsinclude, but are not limited to, BF₄ ⁻, B(CN)₄ ⁻, CH₃BF₃ ⁻, CH₂CHBF₃ ⁻,CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻, N(CN)₂ ⁻, C(CN)₃ ⁻,SCN⁻, SeCN⁻, CuCl₂ ⁺, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc. Relatively speaking,the combination of imidazolium- or sulfonium-based cations and complexhalide anions such as AlCl₄ ⁻, BF₄ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, NTf₂ ⁻,N(SO₂F)₂ ⁻, or F(HF)_(2.3) ⁻ results in RTILs with good workingconductivities.

RTILs can possess archetypical properties such as high intrinsic ionicconductivity, high thermal stability, low volatility, low (practicallyzero) vapor pressure, non-flammability, the ability to remain liquid ata wide range of temperatures above and below room temperature, highpolarity, high viscosity, and wide electrochemical windows. Theseproperties, except for the high viscosity, are desirable attributes whenit comes to using an RTIL as an electrolyte co-solvent in a rechargeablelithium cell.

In the lithium battery, the anode active material may contain, as anexample, lithium metal foil or a high-capacity Si, Sn, or SnO₂ capableof storing a great amount of lithium. For Li—S cells, the cathode activematerial may contain pure sulfur (if the anode active material containslithium), lithium polysulfide, or any sulfur containing compound,molecule, or polymer. If the cathode active material includeslithium-containing species (e.g. lithium polysulfide) when the cell ismade, the anode active material can be any material capable of storing alarge amount of lithium (e.g. Si, Ge, Sn, SnO₂, etc). For other lithiumsecondary cells, the cathode active materials can include a transitionmetal dichalcogenide (e.g., TiS₂, TaS₂, and MoS₂), a transition metaltrichalcogenide (e.g., NbSe₃), a transition metal oxide (e.g., MnO₂,CoO₂, an iron oxide, a vanadium oxide, etc), or a combination thereof.The vanadium oxide may be selected from the group consisting of VO₂,Li_(x)VO₂, V₂O₅, Li_(x)V₂O₅, V₃O₈, Li_(x)V₃O₈, Li_(x)V₃O₇, V₄O₉,Li_(x)V₄O₉, V₆O₁₃, Li_(x)V₆O₁₃, their doped versions, their derivatives,and combinations thereof, wherein 0.1<x<5.

The rechargeable lithium metal or lithium-ion cell featuring an organicliquid solvent-based quai-solid electrolyte containing a high lithiumsalt concentration may contain a cathode active material selected from alayered compound LiMO₂, spinel compound LiM₂O₄, olivine compound LiMPO₄,silicate compound Li₂MSiO₄, Tavorite compound LiMPO₄F, borate compoundLiMBO₃, or a combination thereof, wherein M is a transition metal or amixture of multiple transition metals.

Typically, the cathode active materials are not electrically conducting.Hence, in one embodiment, the cathode active material may be mixed witha conductive filler such as carbon black (CB), acetylene black (AB),graphite particles, expanded graphite particles, activated carbon,meso-porous carbon, meso-carbon micro bead (MCMB), carbon nano-tube(CNT), carbon nano-fiber (CNF), graphene sheet (also referred to as nanographene platelet, NGP), carbon fiber, or a combination thereof. Thesecarbon/graphite/graphene materials may be made into a form of fabric,mat, or paper for supporting the cathode active material.

In a preferred embodiment, the nano-scaled filaments (e.g. CNTs, CNFs,and/or NGPs) are formed into a porous nano-structure that containsmassive surfaces to support either the anode active material (e.g. Li orSi coating) or the cathode active material (e.g. sulfur, lithiumpolysulfide, vanadium oxide, TiS₂, etc). The porous nano-structureshould have pores having a pore size preferably from 2 nm to 1 μm priorto being impregnated with sulfur or lithium polysulfide. The pore sizeis preferably in the range from 2 nm-50 nm, further preferably 2 nm-10nm, after the pores are impregnated with sulfur or lithium polysulfide.These pores are properly sized to accommodate the electrolyte at thecathode side and to retain the cathode active material in the poresduring repeated charges/discharges. The same type of nano-structure maybe implemented at the anode side to support the anode active material.

In another preferred embodiment, the cathode active material consists of(a) exfoliated graphite worms that are interconnected to form a porous,conductive graphite flake network comprising pores having a size smallerthan 100 nm; and (b) nano-scaled powder or coating of sulfur, sulfurcompound, or lithium polysulfide disposed in the pores or coated on agraphite flake surface wherein the powder or coating is in contact withthe electrolyte and has a dimension less than 100 nm. Preferably, theexfoliated graphite worm amount is in the range from 1% to 90% by weightand the amount of powder or coating is in the range from 99% to 10% byweight based on the total weight of exfoliated graphite worms andsulfur, sulfur compound, or lithium polysulfide combined which ismeasured or calculated when the cell is in a fully charged state.Preferably, the amount of the powder or coating of sulfur, sulfurcompound, or lithium polysulfide is in the range from 70% to 95% byweight. Most preferably, the amount of the powder or coating of sulfur,sulfur compound, or lithium polysulfide is no less than 80% by weight.

The electrons coming from or going out through the external load orcircuit must go through the conductive additives (in a conventionalsulfur cathode) or a conductive framework (e.g. exfoliated graphitemeso-porous structure or nano-structure of conductive nano-filaments) toreach the cathode active material. Since the cathode active material(e.g. sulfur, lithium polysulfide, vanadium oxide, etc) is a poorelectronic conductor, the active material particle or coating must be asthin as possible to reduce the required electron travel distance.

Conventional Li—S cells typically have been limited to less than 70% byweight of sulfur in a composite cathode composed of sulfur and theconductive additive/support. Even when the sulfur content in the priorart composite cathode reaches or exceeds 70% by weight, the specificcapacity of the composite cathode is typically significantly lower thanwhat is expected based on theoretical predictions. For instance, thetheoretical specific capacity of sulfur is 1,675 mAh/g. A compositecathode composed of 70% sulfur (S) and 30% carbon black (CB), withoutany binder, should be capable of storing up to 1,675×70%=1,172 mAh/g.Unfortunately, the actually observed specific capacity is typically lessthan 75% (often less than 50%) of what can be achieved. In other words,the active material utilization rate is typically less than 75% (or even<50%). This has been a major issue in the art of Li—S cells and therehas been no solution to this problem. Most surprisingly, theimplementation of exfoliated graphite worms as a conductive supportingmaterial for sulfur or lithium polysulfide, coupled with an ionic liquidelectrolyte at the cathode, has made it possible to achieve an activematerial utilization rate of typically >>80%, more often greater than90%, and, in many cases, close to 99%.

In a preferred lithium-sulfur cell, the pores of the poroussulfur/exfoliated graphite mixture or composite preferably have a sizefrom 2 nm to 10 nm to accommodate electrolyte therein after thenano-scaled powder or coating of sulfur, sulfur compound, or lithiumpolysulfide is disposed in the pores or coated on the graphite flakesurface. These pore sizes in the sulfur/exfoliated graphite mixture orcomposite are surprisingly capable of further suppressing, reducing, oreliminating the shuttle effect. Not wishing to be bound by the theory,but we feel that this is likely due to the unexpected capability ofexfoliated graphite flake surfaces spaced 2-10 nm apart to retainlithium polysulfides in the minute pockets (pores) during the charge anddischarge cycles. This ability of graphitic surfaces to preventout-migration of lithium polysulfide is another big surprise to us.

The exfoliated graphite worms can be obtained from the intercalation andexfoliation of a laminar graphite material. The conventional process forproducing exfoliated graphite worms typically begins with subjecting agraphitic material to a chemical treatment (intercalation and/oroxidation using a strong acid and/or oxidizing agent) to form a graphiteintercalation compound (GIC) or graphite oxide (GO). This is most oftenaccomplished by immersing natural graphite powder in a mixture ofsulfuric acid, nitric acid (an oxidizing agent), and another oxidizingagent (e.g. potassium permanganate or sodium chlorate). The resultingGIC is actually some type of graphite oxide (GO) particles. This GIC isthen repeatedly washed and rinsed in water to remove excess acids,resulting in a graphite oxide suspension or dispersion, which containsdiscrete and visually discernible graphite oxide particles dispersed inwater. There are different processing routes that can be followed afterthis rinsing step to form different types of graphite or grapheneproducts.

For instance, a first route involves removing water from the suspensionto obtain “expandable graphite,” which is essentially a mass of driedGIC or dried graphite oxide particles. Upon exposure of expandablegraphite to a temperature in the range from typically 800-1,050° C. forapproximately 30 seconds to 2 minutes, the GIC undergoes a rapidexpansion by a factor of 30-800 to form “graphite worms”, which are eacha collection of exfoliated, but largely un-separated or stillinterconnected graphite flakes.

As a second route, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes,” which areisolated and separated graphite flakes or platelets thicker than 100 nm(hence, not a nano material by definition). Alternatively, exfoliatedgraphite worms may be the re-compressed (e.g. roll-pressed) to formflexible graphite sheet or flexible graphite foil that is essentially asolid film not permeable to battery electrolyte. Such anelectrolyte-impermeable film can be a good battery current collector(e.g. to replace aluminum foil), but it does not have a sufficientamount of specific surface area to support sulfur.

Alternatively, as a third route, the exfoliated graphite worms may besubjected to high-intensity mechanical shearing (e.g. using anultrasonicator, high-shear mixer, high-intensity air jet mill, orhigh-energy ball mill) to form separated single-layer and/or multi-layergraphene sheets (collectively called nano graphene platelets or NGPs),as disclosed in our U.S. application Ser. No. 10/858,814 (U.S. Pat. Pub.No. 2005-0271574) (now abandoned). Single-layer graphene can be as thinas 0.34 nm, while multi-layer graphene can have a thickness up to 100nm.

The graphite oxide suspension (after a sufficiently high degree ofoxidation) may be subjected to ultrasonication for the purpose ofseparating/isolating individual graphene oxide sheets from graphiteoxide particles. This is based on the notion that the inter-grapheneplane separation has been increased from 0.335 nm in natural graphite to0.6-1.1 nm in highly oxidized graphite oxide, significantly weakeningthe van der Waals forces that hold neighboring planes together.Ultrasonic power can be sufficient to further separate graphene planesheets to form separated, isolated, or discrete graphene oxide (GO)sheets having an oxygen content of typically 20-50% by weight. Thesegraphene oxide sheets can then be chemically or thermally reduced toobtain “reduced graphene oxides” (RGO) typically having an oxygencontent of 0.01%-10% by weight, more typically 0.01%-5% by weight, andmost typically 0.01%-2% by weight.

In general, NGPs include single-layer and multi-layer graphene orreduced graphene oxide with an oxygen content of 0-10% by weight, moretypically 0-5% by weight, and preferably 0-2% weight. Pristine graphenehas essentially 0% oxygen. Graphene oxide (including RGO) can have0.01%-50% by weight of oxygen.

As indicated earlier, dried GIC or GO powder may be exposed a thermalshock (at a high temperature, typically 800-1,050° C.) for a shortperiod of time (typically 30-120 seconds), allowing the constituentgraphite flakes to freely expand. The resulting graphite worms typicallyhave an expanded volume that is 30 to 800 times higher than the originalgraphite volume, depending upon the degree of oxidation orintercalation.

Typically, an oxygen content between 46-50% by weight based on the totalGO weight is an indication of practically complete oxidation ofgraphite, which is also reflected by the complete disappearance of theX-ray diffraction curve peak originally located at 20=approximately 26degrees for un-intercalated or un-oxidized natural graphite. Thisdiffraction peak at 20=approximately 26 degrees corresponds to the d₀₀₂spacing between two (002) graphene planes.

Acids, such as sulfuric acid, are not the only type of intercalatingagent (intercalant) that penetrate into spaces between graphene planes.Many other types of intercalating agents, such as alkali metals (Li, K,Na, Cs, and their alloys or eutectics), can be used to intercalategraphite to stage 1, stage 2, stage 3, etc. Stage n implies oneintercalant layer for every n graphene planes. For instance, a stage-1potassium-intercalated GIC means there is one layer of K for everygraphene plane; or, one can find one layer of K atoms inserted betweentwo adjacent graphene planes in a G/K/G/K/G/KG . . . sequence, where Gis a graphene plane and K is a potassium atom plane. A stage-2 GIC willhave a sequence of GG/K/GG/K/GG/K/GG . . . and a stage-3 GIC will have asequence of GGG/K/GGG/K/GGG . . . , etc.

A graphite worm is characterized as having a network of largelyinterconnected exfoliated graphite flaks with pores between flakes. Theflakes have a typical length or width dimension of 0.5-100 μm (moretypically 1-20 μm), depending upon the types of starting graphiticmaterials used and these lateral dimensions (length or width) arerelatively independent of the GIC stage number (or oxygen content inGO), the exfoliation temperature, and the exfoliation environment.However, these factors have major impact on the volume expansion ratio(exfoliated graphite worm volume vs. starting graphite particle volume),flake thickness range, and pore size range of exfoliated graphite worms.

For instance, Stage-1 GIC or fully oxidized graphite (GO with 40-50%oxygen content), upon un-constrained exfoliation at 1,000° C. for oneminute, exhibit a typical volume expansion ratio of approximately450-800%, flake thickness range of 0.34 to 3 nm, and pore size range of50 nm to 20 μm. By contrast, Stage-5 GIC or GO with 20-25% oxygencontent, upon un-constrained exfoliation at 1,000° C. for one minute,exhibit a volume expansion ratio of approximately 80-180%, flakethickness range of 1.7 to 200 nm, and pore size range of 30 nm to 2 μm.

Stage-1 GIC is the most desirable since it leads to highly exfoliatedgraphite worms featuring thin graphite flakes with very high specificsurface areas (typically >500 m²/g, often >700 m²/g, and even >1,000m²/g in several cases). Higher surface areas make it possible to depositthinner sulfur or lithium polysulfide coating given the same sulfur orlithium polysulfide volume. Consequently, there is significantly reducedproportion of thicker coating of sulfur or lithium polysulfide attachedto the exfoliated graphite flake surfaces. This could allow most of thesulfur to be accessible to the lithium ions during the cell discharge.

The flakes in an exfoliated graphite worm remain substantiallyinterconnected (physically in contact with each other or bonded to eachother), forming a network of electron-conducting paths. Hence, theelectrical conductivity of the graphite worms is relatively high(10-10,000 S/cm), which can be orders of magnitude higher than that ofcarbon black, activated carbon, polymeric carbon, amorphous carbon, hardcarbon, soft carbon, and meso-phase pitch, etc.

The soft and fluffy worms, upon impregnation or coating with sulfur,have exhibited an unexpected improvement in mechanical strength (e.g.compression strength or bending strength) by up to 2-3 orders ofmagnitude. The impregnated graphite worms may be re-compressed toincrease their physical density and structural integrity, if deemednecessary. Graphite worm-sulfur composites have a density typically inthe range from 0.02 g/cm³ to 1.0 g/cm³, depending upon the degree ofexfoliation and the condition of re-compression.

When the cathode is made, the cathode active material (sulfur, lithiumpolysulfide, vanadium oxide, titanium disulfide, etc.) is embedded inthe nano-scaled pores constituted by the exfoliated graphite flakes.Preferably, the cathode active material is grinded into nanometer scale(preferably <10 nm and more preferably <5 nm). Alternatively, thecathode active material may be in a thin-film coating form deposited onsurfaces of the graphite flakes obtained by melt impregnation, solutiondeposition, electro-deposition, chemical vapor deposition (CVD),physical vapor deposition, sputtering, laser ablation, etc. This coatingis then brought in contact with electrolyte before, during, or after thecathode is made, or even after the cell is produced.

The present design of a meso-porous graphite worm cathode withmeso-scaled pores in a Li—S cell was mainly motivated by the notion thata significant drawback with cells containing cathodes comprisingelemental sulfur, organosulfur and carbon-sulfur materials is related tothe dissolution and excessive out-diffusion of soluble sulfides,polysulfides, organo-sulfides, carbon-sulfides and/orcarbon-polysulfides (anionic reduction products) from the cathode intothe rest (anode, in particular) of the cell. This process leads toseveral problems: high self-discharge rates, loss of cathode capacity,corrosion of current collectors and electrical leads leading to loss ofelectrical contact to active cell components, fouling of the anodesurface giving rise to malfunction of the anode, and clogging of thepores in the cell membrane separator which leads to loss of iontransport and large increases in internal resistance in the cell.

At the anode side, when lithium metal is used as the sole anode activematerial in a Li metal cell, there is concern about the formation oflithium dendrites, which could lead to internal shorting and thermalrunaway. Herein, we have used two approaches, separately or incombination, to addressing this dendrite formation issue: one involvingthe use of a high-concentration electrolyte and the other the use of anano-structure composed of conductive nano-filaments. For the latter,multiple conductive nano-filaments are processed to form an integratedaggregate structure, preferably in the form of a closely packed web,mat, or paper, characterized in that these filaments are intersected,overlapped, or somehow bonded (e.g., using a binder material) to oneanother to form a network of electron-conducting paths. The integratedstructure has substantially interconnected pores to accommodateelectrolyte. The nano-filament may be selected from, as examples, acarbon nano fiber (CNF), graphite nano fiber (GNF), carbon nano-tube(CNT), metal nano wire (MNW), conductive nano-fibers obtained byelectro-spinning, conductive electro-spun composite nano-fibers,nano-scaled graphene platelet (NGP), or a combination thereof. Thenano-filaments may be bonded by a binder material selected from apolymer, coal tar pitch, petroleum pitch, meso-phase pitch, coke, or aderivative thereof.

Surprisingly and significantly, the nano-structure provides anenvironment that is conducive to uniform deposition of lithium atoms, tothe extent that no geometrically sharp structures or dendrites werefound in the anode after a large number of cycles. Not wishing to bebound by any theory, but the applicants envision that the 3-D network ofhighly conductive nano-filaments provide a substantially uniformattraction of lithium ions back onto the filament surfaces duringre-charging. Furthermore, due to the nanometer sizes of the filaments,there is a large amount of surface area per unit volume or per unitweight of the nano-filaments. This ultra-high specific surface areaoffers the lithium ions an opportunity to uniformly deposit a lithiummetal coating on filament surfaces at a high rate, enabling highre-charge rates for a lithium metal secondary battery.

The presently invented high-concentration electrolyte andelectrolyte-separator system and optional meso-porous exfoliatedgraphite-sulfur may be incorporated in several broad classes ofrechargeable lithium cells. In the following examples, sulfur or lithiumpolysulfide is used as a cathode active material for illustrationpurposes:

-   -   (A) Lithium metal-sulfur with a conventional anode        configuration: The cell contains an optional cathode current        collector, a cathode (containing a composite of sulfur or        lithium polysulfide and a conductive additive or a conductive        supporting framework, such as a meso-porous exfoliated graphite        or a nano-structure of conductive nano-filaments), a        separator/electrolyte (featuring the gradient electrolyte        system), and an anode current collector. Potential dendrite        formation may be overcome by using the high-concentration        electrolyte at the anode.    -   (B) Lithium metal-sulfur cell with a nano-structured anode        configuration: The cell contains an optional cathode current        collector, a cathode (containing a composite of sulfur or        lithium polysulfide and a conductive additive or a conductive        supporting framework, such as a meso-porous exfoliated graphite        or a nano-structure of conductive nano-filaments), a        separator/electrolyte (featuring the gradient electrolyte        system), an optional anode current collector, and a        nano-structure to accommodate lithium metal that is deposited        back to the anode during a charge or re-charge operation. This        nano-structure (web, mat, or paper) of nano-filaments provide a        uniform electric field enabling uniform Li metal deposition,        reducing the propensity to form dendrites. This configuration,        coupled with the high-concentration electrolyte at the anode,        provides a dendrite-free cell for a long and safe cycling        behavior.    -   (C) Lithium ion-sulfur cell with a conventional anode: For        instance, the cell contains an anode composed of anode active        graphite particles bonded by a binder, such as polyvinylidene        fluoride (PVDF) or styrene-butadiene rubber (SBR). The cell also        contains a cathode current collector, a cathode (containing a        composite of sulfur or lithium polysulfide and a conductive        additive or a conductive supporting framework, such as a        meso-porous exfoliated graphite or a nano-structure of        conductive nano-filaments), a separator/electrolyte (featuring        the quasi-solid electrolyte system), and an anode current        collector; and    -   (D) Lithium ion-sulfur cell with a nano-structured anode: For        instance, the cell contains a web of nano-fibers coated with Si        coating or bonded with Si nano particles. The cell also contains        an optional cathode current collector, a cathode (containing a        composite of sulfur or lithium polysulfide and a conductive        additive or a conductive supporting framework, such as a        meso-porous exfoliated graphite or a nano-structure of        conductive nano-filaments), a separator/electrolyte (featuring        the quasi-solid electrolyte system), and an anode current        collector. This configuration provides an ultra-high capacity,        high energy density, and a safe and long cycle life.

This sulfur or lithium polysulfide in (A)-(D) can be replaced with anyother type of cathode active materials, such as a transition metaldichalcogenide (e.g., TiS₂), transition metal trichalcogenide (e.g.,NbSe₃), transition metal oxide (e.g., MnO₂, a vanadium oxide, etc), alayered compound LiMO₂, spinel compound LiM₂O₄, olivine compound LiMPO₄,silicate compound Li₂MSiO₄, Tavorite compound LiMPO₄F, borate compoundLiMBO₃, or a combination thereof, wherein M is a transition metal or amixture of multiple transition metals. The cathode active material maybe selected from a metal oxide, a metal oxide-free inorganic material,an organic material, a polymeric material, sulfur, selenium, lithiumpolysulfide, lithium polyselenide, or a combination thereof.

In the lithium-ion sulfur cell (e.g. as described in (C) and (D) above),the anode active material can be selected from a wide range ofhigh-capacity materials, including (a) silicon (Si), germanium (Ge), tin(Sn), lead (Pb), phosphorus(P), antimony (Sb), bismuth (Bi), zinc (Zn),aluminum (Al), nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti),iron (Fe) and cadmium (Cd), and lithiated versions thereof; (b) alloysor intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd withother elements, and lithiated versions thereof, wherein said alloys orcompounds are stoichiometric or non-stoichiometric; (c) oxides,carbides, nitrides, sulfides, phosphides, selenides, and tellurides ofSi, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and theirmixtures or composites, and lithiated versions thereof; (d) salts andhydroxides of Sn and lithiated versions thereof; (e) carbon or graphitematerials and prelithiated versions thereof; and combinations thereof.Non-lithiated versions may be used if the cathode side contains lithiumpolysulfides or other lithium sources when the cell is made.

A possible lithium metal cell may be comprised of an anode currentcollector, an electrolyte phase (optionally but preferably supported bya porous separator, such as a porous polyethylene-polypropyleneco-polymer film), a meso-porous exfoliated graphite worm-sulfur cathodeof the instant disclosure (containing a cathode active material), and anoptional cathode collector. This cathode current collector is optionalbecause the presently invented meso-porous exfoliated graphitestructure, if properly designed, can act as a current collector or as anextension of a current collector.

To achieve high capacity in a battery, it is desirable to have either ahigher quantity or loading of the cathode active material or,preferably, a higher-capacity cathode active material in the cathodelayer. Lithium and sulfur are highly desirable as the electrochemicallyactive materials for the anode and cathode, respectively, because theyprovide nearly the highest energy density possible on a weight or volumebasis of any of the known combinations of active materials (other thanthe Li-air cell). To obtain high energy densities, the lithium can bepresent as the pure metal, in an alloy (in a lithium-metal cell), or inan intercalated form (in a lithium-ion cell), and the sulfur can bepresent as elemental sulfur or as a component in an organic or inorganicmaterial with a high sulfur content.

With sulfur-based compounds, which have much higher specific capacitiesthan the transition metal oxides of lithium-ion cells, it is difficultto achieve efficient electrochemical utilization of the sulfur-basedcompounds at high volumetric densities because the sulfur-basedcompounds are highly insulating and are generally not micro-porous. Forexample, U.S. Pat. No. 5,532,077 to Chu describes the problems ofovercoming the insulating character of elemental sulfur in compositecathodes and the use of a large volume fraction of an electronicallyconductive material (carbon black) and of an ionically conductivematerial (e.g., polyethylene oxide or PEO) in the composite electrode totry to overcome these problems. Typically, Chu had to use nearly 50% ormore of non-active materials (e.g., carbon black, binder, PEO, etc),effectively limiting the relative amount of active sulfur. Furthermore,presumably one could choose to use carbon paper (instead of or inaddition to carbon black) as a host for the cathode active material.However, this conventional carbon fiber paper does not allow asufficient amount of cathode active material to be coated on thelarge-diameter carbon fiber surface yet still maintaining a low coatingthickness, which is required of a reduced lithium diffusion path lengthfor improved charge/discharge rates and reduced resistance. In otherwords, in order to have a reasonable proportion of an electrode activematerial coated on a large-diameter fiber, the coating thickness has tobe proportionally higher. A thicker coating would mean a longerdiffusion path for lithium to come in and out, thereby slowing down thebattery charge/discharge rates. The instant application solved thesechallenging problems by using an integrated 3-D meso-porous graphiteworm structure consisting of nano-thickness exfoliated graphite flakeshaving massive conductive surfaces to host the cathode active material(sulfur, sulfur-containing compound, or lithium polysulfide).

As opposed to carbon paper (often used as a host for elemental sulfur,conductive additives, ion conductors, and electrolyte) that was composedof micron-scaled carbon fibers (typically having a diameter of >12 μm),the instant application makes use of graphite worms of nano-thicknessflakes with a thickness less than 200 nm, preferably and more typicallyless than 100 nm, even more preferably and more typically less than 10nm, and most preferably and more typically less than 3 nm. Theexfoliated graphite worms have been ignored or overlooked by the workersin the art of designing electrodes likely due to the notion that theseworms are perceived as too weak to be handled in an electrode-makingprocess and too weak to support any sulfur-containing electrode activematerial. Indeed, graphite worms are extremely weak. However,impregnation of coating of graphite worms with sulfur or sulfurcompounds significantly enhances the mechanical strength of graphiteworms, to the extent that the resulting composite materials can bereadily formed into a cathode using a conventional batteryelectrode-making machine (coater). Further, there has been no teachingthat exfoliated graphite worms could be used to confine lithiumpolysulfide and preventing lithium polysulfide from migrating out of thecathode and entering the anode. This was not trivial or obvious to oneof ordinary skills in the art.

The interconnected network of exfoliated graphite worms forms acontinuous path for electrons, resulting in significantly reducedinternal energy loss or internal heating for either the anode or thecathode (or both). This network is electronically connected to a currentcollector and, hence, all graphite flakes that constitute graphite wormsare essentially connected to the current collector. In the instantdisclosure, the lithium sulfide coating is deposited on flake surfacesand, even if the coating were to fracture into separate segments,individual segments would still remain in physical contact with theunderlying flakes, which is essentially part of the current collector.The electrons transported to the cathode can be distributed to allcathode active coatings. In the case of lithium sulfide particlesdispersed/dissolved in an electrolyte inside meso pores of the cathodestructure, the particles are necessarily nano-scaled (thesalt-electrolyte solution pool also nano-scaled) and, hence, areconducive to fast cathode reaction during the charging operation.

The lithium metal cell of the instant application can have anano-structured anode or a more conventional anode structure, althoughsuch a conventional structure is not preferred. In a more conventionalanode structure, acetylene black, carbon black, or ultra-fine graphiteparticles may be used as a conductive additive. The binder may be chosenfrom polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF),ethylene-propylene-diene copolymer (EPDM), or styrene-butadiene rubber(SBR), for example. Conductive materials such as electronicallyconductive polymers, meso-phase pitch, coal tar pitch, and petroleumpitch may also be used as a binder. Preferable mixing ratio of theseingredients may be 80 to 95% by weight for the anode active material(natural or artificial graphite particles, MCMBs, coke-based anodeparticles, carbon-coated Si nano particles, etc.), 3 to 20% by weightfor the conductive additive, and 2 to 7% by weight for the binder. Theanode current collector may be selected from copper foil or stainlesssteel foil. The cathode current collector may be an aluminum foil or anickel foil. There is no particularly significant restriction on thetype of current collector, provided the material is a good electricalconductor and relatively corrosion resistant. The separator may beselected from a polymeric nonwoven fabric, porous polyethylene film,porous polypropylene film, or porous PTFE film.

The most important property of a filament herein used to support anelectrode active material (e.g. Li or Si at the anode) is a highelectrical conductivity to enable facile transport of electrons withminimal resistance. A low conductivity implies a high resistance andhigh energy loss, which is undesirable. The filament should also bechemically and thermo-mechanically compatible with the intended activematerial (i.e., lithium at the anode) to ensure a good contact betweenthe filament and the coating upon repeated charging/discharging andheating/cooling cycles. Several techniques can be employed to fabricatea conductive aggregate of filaments (a web or mat), which is amonolithic body having desired interconnected pores. In one preferredembodiment of the present disclosure, the porous web can be made byusing a slurry molding or a filament/binder spraying technique. Thesemethods can be carried out in the following ways:

EXAMPLES

In the examples discussed below, unless otherwise noted, raw materialssuch as silicon, germanium, bismuth, antimony, zinc, iron, nickel,titanium, cobalt, and tin were obtained from either Alfa Aesar of WardHill, Mass., Aldrich Chemical Company of Milwaukee, Wis. or Alcan MetalPowders of Berkeley, Calif. X-ray diffraction patterns were collectedusing a diffractometer equipped with a copper target x-ray tube and adiffracted beam monochromator. The presence or absence of characteristicpatterns of peaks was observed for each of the alloy samples studied.For example, a phase was considered to be amorphous when the X-raydiffraction pattern was absent or lacked sharp, well-defined peaks. Inseveral cases, scanning electron microscopy (SEM) and transmissionelectron microscopy (TEM) were used to characterize the structure andmorphology of the hybrid material samples.

A nano-structured cathode, comprising exfoliated graphite worm-sulfur(or polysulfide), was bonded onto an aluminum foil (a currentcollector). After solvent removal, web-aluminum foil configuration washot-pressed to obtain a cathode or, alternatively, a complete cell wasfabricated by laminating an anode current collector (Cu foil), an anodelayer (e.g., a piece of Li foil, a nano-structured web with Si coating,or graphite particles bonded by PVDF), an electrolyte-separator layer, ameso-porous cathode, and a cathode current collector (e.g., stainlesssteel foil or aluminum foil) all at the same time. In some cases, anNGP-containing resin was used as a binder, for instance, between acathode layer and a cathode current collector. Filaments may also bebonded by an intrinsically conductive polymer as a binder to form a web.For instance, polyaniline-maleic acid-dodecyl hydrogensulfate salt maybe synthesized directly via emulsion polymerization pathway usingbenzoyl peroxide oxidant, sodium dodecyl sulfate surfactant, and maleicacid as dopants. Dry polyaniline-based powder may be dissolved in DMF upto 2% w/v to form a solution.

The conventional cathode of a Li—S cell was prepared in the followingway. As an example, 60-80% by weight of lithium sulfide powder, 3.5% byweight of acetylene black, 13.5-33.5% by weight of graphite, and 3% byweight of ethylene-propylene-diene monomer powder were mixed togetherwith toluene to obtain a mixture. The mixture was then coated on analuminum foil (30 μm) serving as a current collector. The resultingtwo-layer aluminum foil-active material configuration was thenhot-pressed to obtain a positive electrode. In the preparation of aconventional cylindrical cell, a positive electrode, a separatorcomposed of a porous polyethylene film, and a negative electrode wasstacked in this order. The stacked body was spirally wound with aseparator layer being disposed at the outermost side to obtain anelectrode assembly. The battery cell is typically configured to leave anopen channel when the cell is made without liquid electrolyte. Liquidelectrolyte is then injected into the resulting battery cell throughsuch a channel in a dry room with a tightly controlled moisture level(typically <5% humidity, which is expensive to maintain). Li-ion cellsare similarly made wherein, for instance, the cathode is prepared bymixing 90% by weight of a selected cathode active material with 5%conductive additive (e.g. carbon black), and 5% binder (e.g. PVDF). Incontrast, the presently invented quasi-solid electrolyte-separatortechnology eliminates the need to leave an open channel and makes itpossible to laminate the entire battery cell directly into a highlycompact structure. There is no tendency for electrolyte leakage sincethe electrolyte has a solid-like flow behavior (does not flow much atall).

The following examples are presented primarily for the purpose ofillustrating the best mode practice of the present disclosure, not to beconstrued as limiting the scope of the present disclosure.

Example 1: Some Examples of Electrolytes and Electrolyte-SeparatorLayers Used

A wide range of lithium salts can be used as the lithium salt dissolvedin an organic liquid solvent (alone or in a mixture with another organicliquid or an ionic liquid). The following are good choices for lithiumsalts that tend to be dissolved well in selected organic or ionic liquidsolvents: lithium borofluoride (LiBF₄), lithiumtrifluoro-methanesulfonate (LiCF₃SO₃), lithium bis-trifluoromethylsulfonylimide (LiN(CF₃SO₂)₂ or LITFSI), lithium bis(oxalato)borate(LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), and lithiumbisperfluoroethy-sulfonylimide (LiBETI). A good electrolyte additive forhelping to stabilize Li metal is LiNO₃. Particularly useful ionicliquid-based lithium salts include: lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

Preferred organic liquid solvents include: ethylene carbonate (EC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), propylene carbonate (PC), acetonitrile (AN), vinylene carbonate(VC), allyl ethyl carbonate (AEC), 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),Poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), hydrofloroether (e.g. TPTP),sulfone, and sulfolane.

Preferred ionic liquid solvents may be selected from a room temperatureionic liquid (RTIL) having a cation selected from tetraalkylammonium,di-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium, ordialkylpiperidinium. The counter anion is preferably selected from BF₄⁻, B(CN)₄ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, orN(SO₂F)₂ ⁻. Particularly useful ionic liquid-based solvents includeN-n-butyl-N-ethylpyrrolidinium bis(trifluoromethane sulfonyl)imide(BEPyTFSI), N-methyl-N-propylpiperidinium bis(trifluoromethylsulfonyl)imide (PP₁₃TFSI), and N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide.

Separator films can be any porous polymer films. Of particular use arepolyolefin-based porous films, such as polyethylene (PE), polypropylene(PP), and PE-PP copolymers. Another class of highly desirable separatorfilms is the fabric, mat, or paper made of polymer fibers, glass fibers,ceramic fibers, or a combination thereof. The nano fibers made fromelectro-spinning into a mat form is of particular interest.Electro-spinning parameters can be adjusted to produce various desiredpore sizes.

Example 2: Vapor Pressure of Some Solvents and Corresponding Quasi-SolidElectrolytes with Various Lithium Salt Molecular Ratios

Vapor pressures of several solvents (DOL, DME, PC, AN, with or withoutan ionic liquid-based co-solvent, PP₁₃TFSI) before and after adding awide molecular ratio range of lithium salts, such as lithiumborofluoride (LiBF₄), lithium trifluoro-methanesulfonate (LiCF₃SO₃), orbis(trifluoro methanesulfonyl)imide (LiTFSI), were measured. Some of thevapor pressure ratio data (p_(s)/p=vapor pressure of solution/vaporpressure of solvent alone) are plotted as a function of the lithium saltmolecular ratio x, as shown in FIG. 2-FIG. 5, along with a curverepresenting the Raoult's Law. In all cases, the vapor pressure ratiofollows the theoretical prediction based on Raoult's Law for up tox<0.15 only, above which the vapor pressure deviates from Raoult's Lawin a novel and unprecedented manner. It appears that the vapor pressuredrops at a very high rate when the molecular ratio x exceeds 0.2, andrapidly approaches a minimal or essentially zero when x exceeds 0.4.With a very low pip value, the vapor phase of the electrolyte eithercannot ignite or cannot sustain a flame for longer than 3 seconds onceinitiated.

Example 3: Flash Points and Vapor Pressure of Some Solvents andCorresponding Quasi-Solid Electrolytes with a Lithium Salt MolecularRatio of x=0.3

The flash points and vapor pressures of several solvents and theirelectrolytes with a lithium salt molecular ratio x=0.3 are presented inTable 1 below. It may be noted that, according to the OSHA (OccupationalSafety & Health Administration) classification, any liquid with a flashpoint below 38.7° C. is flammable. However, in order to ensure safety,we have designed our quasi-solid electrolytes to exhibit a flash pointsignificantly higher than 38.7° C. (by a large margin, e.g. at leastincreased by 50° and preferably above 150° C.). The data in Table 1indicate that the addition of a lithium salt to a molecular ratio of0.35 is normally sufficient to meet these criteria.

TABLE 1 The flash points and vapor pressures of select solvents andtheir electrolytes with a lithium salt molecular ratio x = 0.3. Flashpoint (° C.) with Vapor pressure Flash point x = 0.35 of Vapor pressure(kPa) at 20° C. Chemical (° C.) (Li salt) (kPa) at 20° C. with x = 0.35Acetone −17 — 24 kPa (240 hPa) — Ethanol 17 — — Gasoline −42 — — Canolaoil 327 — — DOL (1,3-dioxolane) 1 75 (LiBF₄) 9.33 (70 Torr) 2.6 (LiBF₄)DOL 1 155 (LiCF₃SO₃) 9.33 0.8 (LiCF₃SO₃) DEC (diethyl carbonate) 33 >200(LiCF₃SO₃) 1.33 (10 Torr) 0.09 (LiCF₃SO₃) DMC (Dimethyl carbonate) 18177 (LiCF₃SO₃) 2.40 (18 Torr) 0.13 (LiCF₃SO₃) EMC (ethyl methylcarbonate) 23 188 (LiBOB) 3.60 (27 Torr) 0.1 (LiBOB) EC (ethylenecarbonate) 145 No flash point <0.0013 (0.02 Torr at 36.4° C.) <0.01(LiBOB) (LiBOB) PC (propylene carbonate) 132 No flash point 0.0173 (0.13Torr) <0.01 (LiBOB) (LiBOB) γ-BL (gamma-butyrolactone), 98 No flashpoint 0.20 (1.5 Torr) <0.01 (LiBOB) (LiBOB) AN (Acetonitrile) 6 85(LiBF₄) 9.71 (88.8 Torr at 25° C.) 1.5 (LiBF₄) EA (Ethyl acetate) + DOL−3 70 (LiBF₄) 9.73 1.3 (LiBF₄) DME (1,2-dimethoxyethane) −2 66 (LiPF₆)6.40 (48 Torr) 2.1 (LiPF₆) VC (vinylene carbonate) 53.1 155 (LiPF₆)11.98 (89.9 Torr) 0.9 (LiPF₆) TEGDME (tetraethylene 141 No flash point<0.0013 (<0.01 Torr) < 0.001 glycol dimethylether) (LiPF₆) FEC (Fluoroethylene 122 No flash point 0.021 <0.01 carbonate) (LiPF₆) FPC(Trifluoro propylene No flash point No flash point — — carbonate)(LiPF₆) HFEs (TPTP) hydrofluoro No flash point No flash point 0.7 <0.1ether (LiPF₆) MFE (methyl nonafluorobutyl No flash point No flash point— — ether) (LiPF₆) IL (1-ethyl-3-methyl 283 No flash point — —imadazolium TFSI) (LiTFSI) * As per OSHA (Occupational Safety & HealthAdministration) classification, any liquid with a flash point below38.7° C. is flammable. ** 1 standard atmosphere = 101,325 Pa = 101.325kPa = 1,013.25 hPa. 1 Torr = 133.3 Pa = 0.1333 kPa

Example 4: Lithium Ion Transference Numbers in Several Electrolytes

The Li⁺ ion transference numbers of several types of electrolytes (e.g.LiTFSI salt/(EMImTFSI+DME) solvents) in relation to the lithium saltmolecular ratio were studied and representative results are summarizedin FIG. 6-FIG. 9. In general, the Li⁺ ion transference number in lowsalt concentration electrolytes decreases with increasing concentrationfrom x=0 to x=0.2-0.35. However, beyond molecular ratios of x=0.2-0.35,the transference number increases with increasing salt concentration,indicating a fundamental change in the Li⁺ ion transport mechanism. Thiswas explained in the theoretical sub-sections earlier. When Li⁺ ionstravel in a low salt concentration electrolyte (e.g. x<0.2), a Li⁺ ioncan drags multiple solvating anions or molecules along with it. Thecoordinated migration of such a cluster of charged species can befurther impeded if the fluid viscosity is increased due to more saltdissolved in the solvent. In contrast, when an ultra-high concentrationof lithium salt with x>0.2 is present, Li⁺ ions could significantlyout-number the available solvating anions that otherwise could clusterthe lithium ions, forming multi-ion complex species and slowing downtheir diffusion process. This high Li⁺ ion concentration makes itpossible to have more “free Li⁺ ions” (non-clustered), thereby providinga higher Li⁺ transference number (hence, a facile Li⁺ transport). Thelithium ion transport mechanism changes from a multi-ioncomplex-dominating one (with an overall larger hydrodynamic radius) tosingle ion-dominating one (with a smaller hydrodynamic radius) having alarge number of available free Li⁺ ions. This observation has furtherasserted that an adequate number of Li⁺ ions can quickly move through orfrom the quasi-solid electrolytes to make themselves readily availableto interact or react with a cathode (during discharge) or an anode(during charge), thereby ensuring a good rate capability of a lithiumsecondary cell. Most significantly, these highly concentratedelectrolytes are non-flammable and safe. Combined safety, facile lithiumion transport, and electrochemical performance characteristics have beenthus far difficult to come by for all types of lithium secondarybattery.

Example 5: Exfoliated Graphite Worms from Natural Graphite Using HummersMethod

Graphite intercalation compound (GIC) was prepared by intercalation andoxidation of natural graphite flakes (original size of 200 mesh, fromHuadong Graphite Co., Pingdu, China, milled to approximately 15 μm) withsulfuric acid, sodium nitrate, and potassium permanganate according tothe method of Hummers [U.S. Pat. No. 2,798,878, Jul. 9, 1957]. In thisexample, for every 1 gram of graphite, we used a mixture of 22 ml ofconcentrated sulfuric acid, 2.8 grams of potassium permanganate, and 0.5grams of sodium nitrate. The graphite flakes were immersed in themixture solution and the reaction time was approximately three hours at30° C. It is important to caution that potassium permanganate should begradually added to sulfuric acid in a well-controlled manner to avoidoverheat and other safety issues. Upon completion of the reaction, themixture was poured into deionized water and filtered. The sample wasthen washed repeatedly with deionized water until the pH of the filtratewas approximately 5. The slurry was spray-dried and stored in a vacuumoven at 60° C. for 24 hours. The resulting GIC was exposed to atemperature of 1,050° C. for 35 seconds in a quartz tube filled withnitrogen gas to obtain worms of exfoliated graphite flakes.

Example 6: Conductive Web of Filaments from Electro-Spun PAA Fibrils forAnode

Poly (amic acid) (PAA) precursors for spinning were prepared bycopolymerizing of pyromellitic dianhydride (Aldrich) and4,4′-oxydianiline (Aldrich) in a mixed solvent oftetrahydrofurane/methanol (THF/MeOH, 8/2 by weight). The PAA solutionwas spun into fiber web using an electrostatic spinning apparatus. Theapparatus consisted of a 15 kV d.c. power supply equipped with thepositively charged capillary from which the polymer solution wasextruded, and a negatively charged drum for collecting the fibers.Solvent removal and imidization from PAA were performed concurrently bystepwise heat treatments under air flow at 40° C. for 12 h, 100° C. for1 h, 250° C. for 2 h, and 350° C. for 1 h. The thermally cured polyimide(PI) web samples were carbonized at 1,000° C. to obtain a sample with anaverage fibril diameter of 67 nm. Such a web can be used to accommodatesulfur (or lithium polysulfide), vanadium oxide, titanium disulfide,etc., for the cathode and/or as a conductive substrate for an anodeactive material.

Example 7: Preparation of NGP-Based Webs (Webs of NGPs and NGPs+CNFs)for the Anode or Cathode (as a Conductive Nanostructured Support)

The starting natural graphite flakes (original size of 200 mesh, fromHuadong Graphite Co., Pingdu, China) was milled to approximately 15 μm.The intercalation and oxidation chemicals used in the present study,including fuming nitric acid (>90%), sulfuric acid (95-98%), potassiumchlorate (98%), and hydrochloric acid (37%), were purchased fromSigma-Aldrich and used as received.

A reaction flask containing a magnetic stir bar was charged withsulfuric acid (360 mL) and nitric acid (180 mL) and cooled by immersionin an ice bath. The acid mixture was stirred and allowed to cool for 15min, and graphite particles (20 g) were added under vigorous stirring toavoid agglomeration. After the graphite particles were well dispersed,potassium chlorate (110 g) was added slowly over 15 min to avoid suddenincreases in temperature. The reaction flask was loosely capped to allowevolution of gas from the reaction mixture, which was stirred for 48hours at room temperature. On completion of the reaction, the mixturewas poured into 8 L of deionized water and filtered. The slurry wasspray-dried to recover an expandable graphite sample. The dried,expandable graphite sample was quickly placed in a tube furnacepreheated to 1,000° C. and allowed to stay inside a quartz tube forapproximately 40 seconds to obtain exfoliated graphite worms. The wormswere dispersed in water to form a suspension, which was ultrasonicatedwith a power of 60 watts for 15 minutes to obtain separated NGPs.

Approximately half of the NGP-containing suspension was filtered anddried to obtain several paper-like mats. Vapor grown CNFs were thenadded to the remaining half to form a suspension containing both NGPsand CNFs (20%), which was dried and made into several paper-like mats.Approximately 5% phenolic resin binder was used to help consolidate theweb structures in both samples. Such a web can be as a conductivesubstrate for an anode active material.

Example 8: Physical Vapor Deposition (PVD) of Sulfur on Meso-PorousGraphite Worm Conductive Structures for Li—S Cathodes

In a typical procedure, a meso-porous graphite worm structure or anano-filament web is sealed in a glass tube with the solid sulfurpositioned at one end of the glass tube and the web near another end ata temperature of 40-75° C. The sulfur vapor exposure time was typicallyfrom several minutes to several hours for a sulfur coating of severalnanometers to several microns in thickness. A sulfur coating thicknesslower than 100 nm is preferred, but more preferred is a thickness lowerthan 20 nm, and most preferred is a thickness lower than 10 nm (or even5 nm). Several lithium metal cells with or without a nano-structuredanode were fabricated, wherein a lithium metal foil was used as a sourceof Li⁺ ions.

Example 9: Preparation of Graphene-Enabled Li_(x)V₃O₈ Nano-Sheets (as aCathode Active Material in a Rechargeable Lithium Metal Battery) fromV₂O₅ and LiOH

All chemicals used in this study were analytical grade and were used asreceived without further purification. V₂O₅ (99.6%, Alfa Aesar) and LiOH(99+%, Sigma-Aldrich) were used to prepare the precursor solution.Graphene oxide (GO, 1% w/v obtained in Example 2 above) was used as astructure modifier. First, V₂O₅ and LiOH in a stoichiometric V/Li ratioof 1:3 were dissolved in actively stirred de-ionized water at 50° C.until an aqueous solution of Li_(x)V₃O₈ was formed. Then, GO suspensionwas added while stirring, and the resulting suspension was atomized anddried in an oven at 160° C. to produce the spherical compositeparticulates of GO/Li_(x)V₃O₈ nano-sheets. Corresponding Li_(x)V₃O₈materials were obtained under comparable processing conditions, butwithout graphene oxide sheets.

An additional set of graphene-enabled Li_(x)V₃O₈ nano-sheet compositeparticulates was produced from V₂O₅ and LiOH under comparableconditions, but was dried under different atomization temperatures,pressures, and gas flow rates to achieve four samples of compositeparticulates with four different Li_(x)V₃O₈ nano-sheet averagethicknesses (4.6 nm, 8.5 nm, 14 nm, and 35 nm). A sample of Li_(x)V₃O₈sheets/rods with an average thickness/diameter of 76 nm was alsoobtained without the presence of graphene oxide sheets (but, with thepresence of carbon black particles) under the same processing conditionsfor the graphene-enhanced particulates with a nano-sheet averagethickness of 35 nm. It seems that carbon black is not as good anucleating agent as graphene for the formation of Li_(x)V₃O₈ nano-sheetcrystals. The specific capacities and other electrochemical propertiesof these cathode materials in Li metal cells using lithium foil as acounter electrode and in Li-ion cells using a graphite anode wereinvestigated.

Example 10: Hydrothermal Synthesis of Graphene-Enabled V₃O₇ H₂ONano-Belts from V₂O₅ and Graphene Oxide

In a typical procedure, 0.015 g of V₂O₅ was added into 9 ml of distilledwater. A GO-water suspension (V₂O₅/GO ratio of 98/2) was poured into theV₂O₅ suspension. The resulting mixture was transferred to a 35 mlTeflon-sealed autoclave and stored at 180-200° C. for 24-36 h (differentbatches), then was air-cooled to room temperature. GO was used as aheterogeneous nucleation agent to promote fast nucleation of largernumbers of nuclei for reduced crystallite sizes (promote nucleationagainst growth of crystals). The products were washed several times withdistilled water, and finally dried at 60° C. in an oven.

A second batch was obtained by spray-drying at 200° C. and heat-treatedat 400° C. for 2 hours to obtain particulates of GO/V₃O₇ H₂O compositewith graphene oxide sheets embracing around these particulates. Forcomparison purposes, a third batch of V₃O₇ H₂O was prepared withoutusing GO (oven dried), a fourth batch was prepared with GO and polyethylene oxide (1% PEO in water was added to the GO suspension, thenspray-dried and heat-treated at 400° C. for 2 hours), and a fifth batchwas prepared with PEO (1% in water, but without GO) via spray-drying,followed by heat-treating at 400° C. for 2 hours. Heat treatment of PEOat 400° C. serves to convert PEO to a carbon material. The particulatesof GO/V₃O₇ H₂O composite were used as a cathode active material in a Limetal cell.

Example 11: Preparation of Electrodes for Li-Ion Cells Featuring aQuasi-Solid Electrolyte

Several dry electrodes containing graphene-enhanced particulates (e.g.comprising lithium cobalt oxide or lithium iron phosphate primaryparticles embraced by graphene sheets) were prepared by mixing theparticulates with a liquid to form a paste without using a binder suchas PVDF. The paste was cast onto a surface of a piece of glass, with theliquid medium removed to obtain a dry electrode. Another dry electrodewas prepared by directly mixing LiFePO₄ primary particles with graphenesheets in an identical liquid to form a paste without using a binder.Again, the paste was then cast to form a dry electrode. The dryelectrodes were for the evaluation of the effect of various conductiveadditives on the electrical conductivity of an electrode.

For comparison purposes, several additional dried electrodes wereprepared under exactly identical conditions, and the paste in each casewas made to contain the same cathode active particles, but a comparableamount of other conductive additives: multi-walled carbon nano-tubes(CNTs), carbon black (Super-P from Timcal), a CNT/Super-P mixture at an1/1 ratio, and a GO/Super-P mixture at an 1/1 ratio. Corresponding “wet”electrodes for incorporation in a battery cell were made to contain aPVDF binder. These electrodes were made into full cells containinggraphite particles or lithium metal as an anode active material.

The first-cycle discharge capacity data of small full button cellscontaining lithium metal as an anode active material were obtained. Thedata show that the battery cells containing graphene-enhancedparticulates in the cathode show superior rate capability to that of acarbon black-enhanced cathode. Most importantly, the Li-ion cells havinga higher salt concentration in an organic liquid solvent typicallyexhibit a longer and more stable cycling life, experiencing asignificantly lesser extent of capacity decay after a give number ofcharge/discharge cycles.

It may be further noted that the cathode active material that can beused in the presently invented electrode is not limited to lithiumcobalt oxide and lithium iron phosphate. There is no particularlimitation on the type of electrode active materials that can be used ina Li-ion cell featuring the presently invented quasi-solid electrolyte.

Example 12: Li-Air Cells with Ionic Liquid Electrolytes ContainingVarious Salt Concentrations

To test the performance of the Li-air battery employing an organicliquid solvent with different lithium salt concentrations, several pouchcells with dimension of 5 cm×5 cm were built. Porous carbon electrodeswere prepared by first preparing ink slurries by dissolving a 90 wt %EC600JD Ketjen black (AkzoNobel) and 5 wt. % Kynar PVDF (ArkemaCorporation) in N-methyl-2-pyrrolidone (NMP). Air electrodes wereprepared with a carbon loading of approximately 20.0 mg/cm² byhand-painting the inks onto a carbon cloth (PANEX 35, ZoltekCorporation), which was then dried at 180° C. overnight. The totalgeometric area of the electrodes was 3.93 cm². The Li/O₂ test pouchcells were assembled in an argon-filled glove box. The cell consists ofmetallic lithium anode and the air electrode as a cathode, prepared asmentioned above. The copper current collector for anode and the aluminumcurrent collector for cathode were used. A Celgard 3401 separatorseparating the two electrodes was soaked in LiTFSI-DOL/EMITFSI (6/4)solutions (with different LiTFSI salt concentrations) for a minimum of24 hours. The cathode was soaked in the oxygen saturatedEMITFSI-DOL/LiTFSI solution for 24 hours and was placed under vacuum foran hour before being used for the cell assembly. The cell was placed inan oxygen filled glove box where oxygen pressure was maintained at 1atm. Cell charge-discharge was carried out with a battery cycler at thecurrent rate of 0.1 mA/cm² at room temperature. It was found that ahigher lithium salt concentration in a liquid solvent results in ahigher round-trip efficiency for cells (62%, 66%, and 75% for x=0.11,0.21, and 0.32, respectively) and lower capacity decay after a givennumber of charge/discharge cycles (25%, 8%, and 4.8% for cells withx=0.11, 0.21, and 0.32, respectively, after 100 cycles).

Example 13: Evaluation of Electrochemical Performance of Various Cells

Charge storage capacities were measured periodically and recorded as afunction of the number of cycles. The specific discharge capacity hereinreferred to is the total charge inserted into the cathode during thedischarge, per unit mass of the composite cathode (counting the weightsof cathode active material, conductive additive or support, binder, andany optional additive combined, but excluding the current collector).The specific charge capacity refers to the amount of charges per unitmass of the composite cathode. The specific energy and specific powervalues presented in this section are based on the total cell weight. Themorphological or micro-structural changes of selected samples after adesired number of repeated charging and recharging cycles were observedusing both transmission electron microscopy (TEM) and scanning electronmicroscopy (SEM).

As an example, the cycling performance (discharge specific capacity) ofa Li metal-sulfur cell containing a low-concentration electrolyte (1M oflithium salt in an organic liquid) is shown in FIG. 10 (the lowercurve). It is quite clear that the capacity of the cell rapidly decaysas charges and discharges are repeated. This is characteristic ofconventional Li—S cells that have great propensity for sulfur andlithium polysulfide to get dissolved in the electrolyte at the cathodeside. Much of the dissolved sulfur could not be re-deposited to thecathode conductive additive/substrate or the cathode current collectorduring subsequent charges/discharges. Most critically, as time goes onor when charge/discharge cycling continues, some of the dissolvedlithium polysulfide species migrate to the anode side and react with Lito form insoluble products and, hence, these species could not return tothe cathode. These phenomena lead to continuing decay in the batterycapacity.

We proceeded to investigate how the lithium salt concentration wouldaffect the lithium polysulfide dissolution in an organic solvent, and todetermine how concentration changes would impact the thermodynamics andkinetics of the shuttle effect. We immediately encounter some majorchallenges. First, we did not have a wide range of lithium saltconcentrations at our disposal. Most of the lithium salts could not bedissolved in those solvents commonly used in Li-ion or Li—S secondarycells for more than a molar ratio of 0.2-0.3. Second, we quickly came torealize that the viscosity of many organic liquid solvents was alreadyextremely high at room temperature and adding more than 0.2-0.3 molarratio of a lithium salt in such a viscous solid made the resultingmixture look like and behave like a solid. It was next to impossible touse a stirrer to help disperse the solid lithium salt powder in theliquid solvent. Further, a higher solute concentration was generallybelieved to be undesirable since a higher concentration normally wouldresult in a lower lithium ion conductivity in the electrolyte. Thiswould not be conducive to achieving a higher power density, lowerpolarization, and higher energy density (at high charge/dischargerates). We almost gave up, but decided to move forward anyway. Theresearch results have been most surprising.

Contrary to the expectations by electrochemists and battery designersthat a significantly higher lithium salt concentration could not beproduced, we found that a concentration as high as x=0.2-0.6, roughlycorresponding to 3-11 M of a lithium salt in some organic liquid couldbe achieved, if a highly volatile solvent (such as AN or DOL) is addedas a co-solvent first. Once a complete dissolution of a lithium salt ina mixture solvent is attained, we could choose to selectively remove theco-solvent. We were pleasantly surprised to observe that partial orcomplete removal of the more volatile co-solvent upon complete saltdissolution would not result in crystallization or precipitation of thesalt from the organic liquid solvent even though the salt (a solute) wasthen in a highly supersaturated condition.

We have further defied the expectation of battery chemists and engineersthat a higher electrolyte concentration would lead to a lower dischargecapacity. Most surprisingly, the Li—S cells contain ahigher-concentration electrolyte system exhibit not only a generallyhigher energy density but also a dramatically more stable cyclingbehavior and longer cycle life.

As an example, FIG. 10 (upper curve) shows the discharge specificcapacity of a Li metal-sulfur cell containing an organic liquidsolvent-based quasi-solid electrolyte (3.5 M). The cycling performanceis so much better than that of the corresponding cell having a lowersalt concentration as shown in the lower curve of FIG. 10. The specificcapacity of this lower concentration cell decays rapidly as the numberof charge/discharge cycles increases.

FIG. 11 shows the Ragone plots (cell power density vs. cell energydensity) of three Li metal-sulfur cells each having an exfoliatedgraphite worm-sulfur cathode, but the lithium salt concentrations being0.07, 0.24, and 0.35, respectively. Not only the energy density, butalso the power density of a Li—S cell is improved when the organicliquid-based solvent has a higher lithium salt concentration. This iscompletely opposite to the expectations of electrochemists and batterydesigners that (1) organic liquids should not be capable of dissolvingmore than x=0.2 or approximately >3.5 M of lithium salt; and (2) with ahigher salt concentration, the electrolyte viscosity should be evenhigher, making the lithium ions even less mobile with a lower diffusioncoefficient and, hence, leading to a reduced lithium ion migration andreduced power density. A logical question to ask is why a higherelectrolyte concentration (higher than 3.5 M) seems to allow for afacile transport of lithium ions, Li⁺. We will briefly repeat theanswers below:

When lithium ions are formed in the conventional lower-concentrationelectrolyte, the positively charged lithium ions Li⁺ might be associatedwith or surrounded by the solvating anions or molecules that help todissolve or “solvate” the Li⁺ ions. Typically, one Li⁺ ion can beclustered with several (2-4) solvating anions. In other words, when aLi⁺ ion moves, it has to drag along several anions to move with it. Sucha Li⁺ ion transport mechanism would be very sensitive to the variationin electrolyte viscosity, which would increase with increasing lithiumsalt concentration and decreasing temperature. In contrast, with a muchhigher lithium salt concentration, there would be significantly more Li⁺ions than the solvating anions in the electrolyte. Consequently, many ofthe Li⁺ ions become “free” ions when the lithium salt concentration issufficiently high. These free Li⁺ ions could move faster than if theywere clustered with solvating anions as in the electrolyte of a low saltconcentration. It seems that when the lithium salt concentration isgreater than x=0.2, the free Li⁺ ions would significantly outnumber thesolvated Li⁺ ions, and the number of free Li⁺ ions would also be greaterthan the total number of Li⁺ ions (regardless if they are clustered withsolvating anions or not) in an electrolyte having a concentration<x=0.1.

Additional data on the correlation between electrochemical performanceof various Li metal cells and Li-ion cells are presented in Table 2below.

TABLE 2 Examples of the electrolytes and lithium salt concentrationsused. Cathode active Li salt Anode Cathode Capacity loss material CellID Solvent(s) (solute) x active material active material after 50 cyclesutilization rate N-1 TPTP + DOL (LiN(CF₃SO₂)₂ 0.08 Li metal 50% S + 50%CB  60% 54% N-2 (ratio = 7:1) 1.6  43% 57% N-3 0.38 2.6% 64% N-4 0.46 0% 68% K-1 TPTP + TEGDME (LiN(CF₃SO₂)₂ 0.11 Li metal 80% S + 20% EG9.5% 82% K-2 (ratio = 6:1) 0.33 1.1% 87% K-3 0.42  0% 91% K-4 0.51  0%93% P-1 DME LiCF₃SO₃ 0.08 Li metal 80% Li_(x)V₃O₈  10% 68% P-2 0.31sheets + 20% CNT 2.3% 79% P-3 0.41 0.3% 84% Q-1 DME LiBOB 0.15 Lithiated80% TiS₂ + 20% EG 4.6% 81% Q-2 0.21 Si nano 2.4% 87% Q-3 0.26 particles1.5% 89% Q-4 0.33 0.9% 92% R-1 DMC + BEPyTFSI LiTFSI 0.07 Li metal 80%S + 20% NGP  12% 74% R-2 (ratio = 10:1) 0.27 3.4% 83%

Based on the data as summarized in Table 1, one can make the followingsignificant observations:

-   -   (a) The higher-concentration electrolyte results in a        dramatically better battery cycling performance (higher capacity        retention rate or lower capacity loss) and a higher cathode        active material utilization rate, as compared to the        lower-concentration electrolyte.    -   (b) In addition to the implementation of a higher concentration        electrolyte-separator system, a meso-porous or porous        nano-structured cathode supporting network composed of        exfoliation graphite worms (EGW) and conductive nano-filaments,        such as carbon nanotubes (CNTs) and graphene (NGPs), further        improves the cycling stability and active material utilization        rate in a rechargeable Li—S cell. Among these conductive network        structures for sulfur and/or lithium polysulfide, AEGWs appear        to provide the best overall performance.

In summary, the present disclosure provides an innovative, versatile,and surprisingly effective platform materials technology that enablesthe design and manufacture of superior lithium metal and lithium-ionrechargeable batteries. The lithium cell featuring a high-concentrationelectrolyte system exhibits a stable and safe anode (no dendrite-likefeature), high lithium utilization rate, high cathode active materialutilization rate, high specific capacity, high specific energy, highpower density, little or no shuttling effect, and long cycle life.

We have further observed that the electrochemical performance of lithiumsecondary cells containing a quasi-solid electrolyte is relativelyindependent of the battery operating temperature and the cells caneffectively operate in an unusually wide temperature range, extending toa very low temperature and a very high temperature regime. Thistemperature range is the widest among all known lithium batteries.

The presently invented cells can provide a specific energy greater than400 Wh/kg (more typically greater than 500 Wh/kg, often greater than 600Wh/kg, and even achieving an unprecedented 800 Wh/kg in some cases)based on the total cell weight including anode, cathode, electrolyte,separator, current collector, and packaging/housing material weightscombined. This has not been achieved by any prior art approach.

1. A method of producing a non-flammable quasi-solid electrolyte for alithium battery, said method comprising (A) dissolving a lithium salt ina first liquid solvent to obtain a mixture having a first concentrationof lithium salt less than 3.0 M (mole/L), but greater than 0.001M; and(B) removing a portion of said first liquid solvent to obtain saidquasi-solid electrolyte having a final lithium salt concentration higherthan said first concentration.
 2. The method of claim 1, whereinquasi-solid electrolyte exhibits a vapor pressure less than 0.01 kPawhen measured at 20° C., a vapor pressure less than 60% of the vaporpressure of said first liquid solvent alone, a flash point at least 20degrees Celsius higher than a flash point of said first liquid solventalone, a flash point higher than 150° C., or no detectable flash point.3. (canceled)
 4. (canceled)
 5. (canceled)
 6. (canceled)
 7. The method ofclaim 1, wherein said first liquid solvent is selected from the groupconsisting of 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME),tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE),2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylene carbonate (EC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), ethyl propionate, methyl propionate, propylene carbonate (PC),gamma-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA),propyl formate (PF), methyl formate (MF), toluene, xylene, methylacetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC),allyl ethyl carbonate (AEC), a hydrofloroether, a room-temperature ionicliquid solvent, and combinations thereof.
 8. The method of claim 7,wherein said ionic liquid solvent has a cation selected fromtetraalkylammonium, di-, tri-, or tetra-alkylimidazolium,alkylpyridinium, dialkyl-pyrrolidinium, dialkylpiperidinium,tetraalkylphosphonium, trialkylsulfonium, or a combination thereof. 9.The method of claim 7, wherein said ionic liquid solvent has an anionselected from BF₄ ⁻, B(CN)₄ ⁻, CH₃BF₃ ⁻, CH₂CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻,N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻, N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻,AlCl₄ ⁻, F(HF)_(2.3) ⁻, or a combination thereof.
 10. The method ofclaim 1, wherein said lithium salt is selected from lithium perchlorate(LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride(LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethylsulfonylimide (LiBETI), lithiumbis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-basedlithium salt, or a combination thereof.
 11. The method of claim 1,wherein said first liquid solvent further comprises an additive.
 12. Themethod of claim 11, wherein said additive is different in compositionthan said liquid solvent and is selected from Hydrofluoro ether (FIFE),Trifluoro propylene carbonate (FPC), Methyl nonafluorobutyl ether (MFE),Fluoroethylene carbonate (FEC), Tris(trimethylsilyl)phosphite (TTSPi),Triallyl phosphate (TAP), Ethylene sulfate (DTD), 1,3-propane sultone(PS), Propene sultone (PES), Alkylsiloxane (Si—O), Alkyylsilane (Si—C),liquid oligomeric silaxane (—Si—O—Si—), tetraethylene glycoldimethylether (TEGDME), canola oil, or a combination thereof and saidadditive-to-said liquid solvent ratio in said mixture is from 1/95 to99/1 by weight.
 13. The method of claim 1, wherein said first liquidsolvent further comprises a second liquid solvent mixed with said firstsolvent to dissolve said lithium salt and the method further comprisespartially or totally removing said second solvent after the lithium saltis dissolved.
 14. The method of claim 13, wherein said second liquidsolvent is selected from acetone, an alcohol, acetonitrile, anether-type solvent, or a combination thereof.
 15. A method of producinga separator/quasi-solid electrolyte layer for use in a lithium battery,said method comprising: (a) providing an ion-permeable or porousthin-film separator, wherein said separator has a thickness less than500 μm; (b) dissolving a lithium salt in a first liquid solvent toobtain a flowable liquid mixture having a first concentration of lithiumsalt less than 3.0 M (mole/L), but greater than 0.001M; and (c) coatingor impregnating said thin-film separator with said flowable liquidmixture to obtain a separator/electrolyte layer and removing a portionof said first liquid solvent to obtain a separator/quasi-solidelectrolyte having a final lithium salt concentration higher than saidfirst concentration.
 16. The method of claim 15, wherein said finallithium salt concentration is from 3.5 M to 15 M and/or has a molecularratio from 0.2 to 0.9.
 17. The method of claim 15, wherein saidelectrolyte has a lithium ion transference number from 0.4 to 0.99. 18.(canceled)
 19. (canceled)
 20. The method of claim 15, which includes aroll-to-roll process wherein (a) entails continuously or intermittentlyfeeding a porous thin-film separator from a feeder roller and step (c)entails collecting said separator-electrolyte layer on a winding roller.21. The method of claim 15, which includes roll-to-roll process wherein(a) entails continuously or intermittently providing a porous thin-filmseparator sheet from a feeder roller, (c) entails depositing saidelectrolyte onto one or two primary surfaces of said separator sheet orimpregnating pores of said separator sheets with said electrolyte andremoving a portion of said first liquid solvent to form saidseparator-electrolyte layer, and (c) further entails collecting saidseparator-electrolyte layer on a winding roller.
 22. The method of claim15, wherein (c) entails spraying and depositing said lithium salt andsaid first liquid solvent concurrently or sequentially onto one or twoprimary surfaces of said separator sheet to form saidseparator-electrolyte layer.
 23. The method of claim 15, wherein (c)includes removing a portion of said first liquid solvent to increase alithium salt concentration.
 24. The method of claim 15, wherein saidfirst liquid solvent contains a mixture of a volatile organic solventand an ionic liquid or a less volatile organic solvent.
 25. The methodof claim 24, wherein said volatile organic solvent contains anether-type solvent selected from 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, or sulfolane. 26.The method of claim 19, wherein said porous thin-film separator isselected from a porous polymer film, a porous mat, fabric, or paper madeof polymer or glass fibers, or a combination thereof.